Elon Musk’s sales of Tesla stock last week came as little surprise to those who have been following the story of his potential tax bill of $10 billion to $15 billion on stock options granted in 2012. Yet according to accountants, most of his sales don’t appear to be connected with taxes — which could mean he will unload far more stock than expected.

The options on Musk’s 23 million shares expire in August, which is also the deadline for the tax bill due to California and the Internal Revenue Service. Musk started exercising the options Nov. 8. He exercised $2.5 billion in shares and sold $1.1 billion of those exercised options to pay the taxes.

“The shares of common stock were sold solely to satisfy the reporting person’s tax withholding obligations related to the exercise of stock options,” said a footnote to his Securities and Exchange Commission filing for Nov. 8.

Then on Monday, Musk sold another $930 million in shares to pay taxes on options that he exercised on 2.1 million shares. That brings his total options exercises to about $4.6 billion and shares sold to meet tax withholding obligations to $2 billion.

Yet most of last week’s sales were for a different reason. Rather than selling as part of an options exercise, Musk started selling from his existing shares. Accountants said it would be impractical for Musk to use those existing shares to pay the tax on his options, since they carry a much higher tax bill.

Musk’s options are taxed as ordinary income, since they are considered compensation. The combined federal and California rate could be as high as 54%. The strike price on the options is $6.24 per share, and Tesla’s stock price on Monday was over $1,160 a share, so he would pay higher taxes — more than $10 billion on his gain of over $20 billion.

Typically, executives sell the exercised shares immediately after they’re purchased to pay the taxes, in what’s known as a “cashless” exercise. Since the shares are sold immediately, there is no additional capital gains tax owed on the shares sold.

Because Musk’s sales beginning Nov. 9 were straight stock sales with little or no cost basis, he would owe long-term capital gains taxes of up to $1.3 billion. Using those proceeds to pay the options tax would amount to paying taxes twice — once on the capital gains and once on the options.

“It wouldn’t make sense from a tax perspective for him to use those proceeds for the options tax,” said Toby Johnston, partner in charge of the Silicon Valley office of Moss Adams, an accounting, consulting and wealth management firm.

Musk admitted that the regular shares are less tax efficient than selling the option shares. “A careful observer would note that my (low basis) share sale rate significantly exceeds my 10b (high basis) option exercise rate, thus closer to tax maximization than minimization,” he tweeted Sunday.

So why is Musk selling the non-option stock given its relatively high tax cost? Tax experts and Tesla analysts say he will still exercise the options before August, since letting them expire would leave billions on the table, along with added ownership of the company, even after paying the taxes. That means he still has billions in stock to exercise and billions to sell to pay the taxes.

The $5.7 billion and whatever additional non-option shares he sells are straight cash-outs. While he does owe federal capital gains taxes on the sales, he likely won’t have to pay state taxes on the gains, since he is likely now a Texas tax resident. The same rule doesn’t apply to his options taxes, though, since those are considered employee benefits and earned while he was in California.

Accountants say the sales aren’t likely for charity, since he would have simply donated appreciated shares rather than selling and paying a capital gains tax first. He could be using the proceeds for Space X, his privately held space company, or for another private venture. Or he could simply want to take money off the table after years of being stock-rich and cash-poor and borrowing off his stock price to fund his lifestyle. Federal taxes are also likely to increase next year, creating an added incentive if he were already considering a cash-out.

Whatever the reasons, Musk will likely end up selling far more than the $10 billion to $15 billion he needs for taxes. He conducted a Twitter poll Nov. 6 in which he asked his followers whether he should sell 10% of his stock and said he would abide by the results. In the vote, 58% of those who responded said he should sell 10% of his shares, which could mean over $20 billion in sales in total.

“For people at his level, taxes aren’t always the primary driver of investment decisions,” Johnston said. “It still feels like there is a piece missing to the puzzle that we may not know about.”