A token can expose a burn function that reverts, that does nothing, or that quietly sends your tokens to its deployer. All three return success. So the contract never asks the token what it did: it reads total supply and its own balance before and after, and labels the record from the difference.

Supply burn

The token's own total supply fell by exactly the amount destroyed. This is the strongest record available and it is rare.

Dead address

The tokens went to 0x…dEaD, which nobody is known to control. Supply did not move, and the record says so rather than rounding up.

No record at all

If a token's burn function is a transfer somewhere else, the whole transaction reverts and you keep your tokens. There is no weaker proof to fall back to, which is the point.