I can explain it to you. A bug description at the beginning is some observed behaviour that seems to be wrong. Now the process starts of UNDERSTANDING the bug. Once that process has concluded, it will be possible to make a rough guess of how long fixing it will take. Very often, the answer then is a minute or two, unless major rewrites are necessary. So, the problem is you cannot put an upfront bound on how long you need to understand the bug. Understanding can be a long winded process that includes trying to fix the bug in the process.
> A bug description at the beginning is some observed behaviour that seems to be wrong.
Or not. A bug description can also be a ticket from a fellow engineer who knows the problem space deeply and have an initial understanding of the bug, likely cause and possible problems. As always, it depends, and IME the kind of bugs that end up in those "bugathons" are the annoying "yeah I know about it, we need to fix it at some point because it's PITA".