system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when my computer is doing something, the "time remaining" estimate fluctuates so much and is rarely accurate?
gpt
Determining accurately how long the task would take would take a while in itself, so the programmers estimate the length of the task using methods which may not be accurate but which don't risk significantly increasing the length of the actual task.  Additionally, there may be circumstances which simply cannot be known.  An example of that would be network speeds that are fluctuating.  You obviously can't know during a download that half way through the network bandwidth is going to go to crap.

An example of a cheap but inaccurate estimate would be to use the number of files but not their sizes when copying files.  If all the files are similar in size, then this is an acceptable estimate, but if you have lots of small files and a few very large ones, then the estimate is going to be way off.