Easy: $1 invested does not equal $1 in market cap. Not sure where I read this, but there was a post where a guy wrote he invested $5,000 in a coin and it increased the market cap by $4m.
As a buy pulls the latest price up, all of the other shares are worth more, and the opposite is true: for every sale, the price might slip down a bit; in the event of a large sell-off, you end up reaching zero value a lot earlier than everything can be sold off.
Market cap is a bit of a fantasy metric, but has utility - just less so for crypto where you can't make any comparisons to tangible assets, liabilities or even market potential.
[1] https://www.google.com/search?q=market+capitalization+defini...
If the orderbook is thin and supply circulated is plenty, the market cap moves by the tradr