Edit: a lot of investors probably take the 21 million as straight facts and don't bother looking into lost coins so it ought also be reasonable to use that as the cap in some cases.
A lot of investors also know that Satoshi (probably) owns 1 million coins and isn't active since the early days, so it can be assumed that those coins won't wake up. And it's also a pretty common knowledge that around 3.3 million coins are assumed lost.
But market capitalization is a metric that rarely gets used for something productive, most of the time it's just pointless comparing for this value between different assets. It's especially bad when people compare between different classes, like saying "Bitcoin's marketcap is bigger than PayPal".
