Apple (AAPL -1.04%) made headlines this week for becoming the first U.S. company to reach a market cap of $2 trillion. That's already a mind-boggling milestone in itself, but even more so when you zoom out and realize that Apple shares have more than doubled from the March lows -- Apple has added $1 trillion to its total valuation within just a few months.

Here's the thing: Apple's market cap might not have actually hit that historical threshold.

Exterior of an Apple Store in Beijing at dusk

Image source: Apple.

The price threshold might be higher

There's a natural lag associated with a lot of market events due to the timing of regulatory filings. In Apple's case, its latest quarterly filing showed that there were just under 4.28 billion shares outstanding as of July 17. Since a company's market cap is simply calculated as its share price multiplied by total shares outstanding, that translates into a price threshold of $467.77 for Apple to be worth $2 trillion -- which the stock indeed traded above briefly yesterday.

But that share count was from over a month ago, and Apple has been aggressively repurchasing its shares hand over fist for years. While some companies might use buyback programs to merely offset dilution from stock-based compensation, the sheer magnitude of Apple's repurchase program -- the Cupertino tech giant has repurchased $360 billion in stock over the past eight years -- has resulted in a steady reduction in shares outstanding.

AAPL Shares Outstanding Chart

AAPL Shares Outstanding data by YCharts

We don't know how many shares Apple has undoubtedly bought back over the past month. Did the company kick off yet another accelerated share repurchase (ASR) program? Probably not, since it commenced a fresh $6 billion ASR last quarter. Is the company already deploying the $5.5 billion in debt it raised last week for open market purchases? Maybe; Apple has historically funded its buybacks with debt, although tax reform in 2017 changed the corporate finance calculus there.

For context, investors do know that Apple had a little over 4.31 billion shares as of June 27 (its last balance sheet date). That shows Apple repurchased about 37 million shares over the course of about 3 weeks. The point is that nobody outside of Apple knows how much stock the company has bought back, but the price threshold for a $2 trillion market cap is a moving target. It will superficially change again once the Mac maker's 4-for-1 stock split takes effect later this month.

Apple has demonstrated that its rising share price rarely dissuades it from engaging in buyback activity, as management has long expressed confidence that the stock is undervalued. Trading at around 35 times earnings, it's hard to argue that Apple shares are expensive.