Unlike fiat which the amount in your bank account remains what it is, bitcoin in your wallet will constantly fluctuate, meaning that there is chances that you can gain or lose your bitcoin.
OP wasn't referring to losing one's Bitcoin to hackers or anything of that nature. Rather, what they were asking was if there was any situation that would make one's Bitcoin hedl to decrease in its unit. Let's say 0.005 BTC reduces to 0.004 and continues that way until it gets to zero or that it increases beyond 0.005 BTC.
I sort of understood it differently going by the snippet you have in that quote. My understanding left me with the impression that, it’s a quantification based on what value your wallet holds through the various market seasons or price fluctuations.
Although, OP have stated it as described, which is not even close to anything possible. That your coins would decrease in units without you having to make some withdrawal or send some out, that doesn’t happen with bitcoin or any other coin in a wallet out there.
Fiat too follow similar pattern in the sense that, what truly decreases is the value. What that same money could buy in different time frames varies. While inflation might rock fiat, volatility still rocks bitcoin. You don’t lose in unit but, you can either profit or lose in value.