Why the two numbers are different
A wallet app multiplies your balance by the last traded price. That price was set by a small trade. When you sell a large amount into a small pool, every token you sell pushes the price down a little further, so the average price you get is lower than the price on screen. This is called price impact. For big coins with deep pools it is tiny. For new memecoins with a few thousand dollars in the pool, it can wipe out most of a bag's value.
Worked example: a coin shows $1,000 in your wallet, and its pool holds $1,000 on the cash side. Selling the whole bag at once would bring in about $500. Read more about paper value and exit value.
How we work it out
What this does not include: trading fees, network fees, and the effect of other people trading while you sell. Prices change by the second, so treat every figure as a snapshot.