I've definitely seen devs assume too much needs to be done, just like I've seen non-devs ignore key parts of the problem that push up the time. Sometimes it's trying to make a general solution when actually what's needed is someone to sit down with a spreadsheet for a day.
> There is back-and-forth as the estimates are questioned for being too high, almost never for being too low.
I'm sure people will have flashbacks when I say this so sorry to those, but this is the issue addressed with planning poker. The idea being that you all say how hard the task is, without being affected by each other, and discuss when expectations aren't aligned. Someone is probably missing something.
I might think something is simple because I've not realised a complex part of the problem, or because I can see a nicer neater solution.