Render time and ETA does not count the time it takes to do a render dump/snapshot time.
If there are many render dumps/snapshots that contribute to a lot of the time, they are not accounted for in ETA or render time.

ETA also does not take into account CPU %. If CPU is run at 1% for an hour, then changed to 100%, its ETA will remain huge and slowly crawl down towards a reasonable estimate, while always being an over estimate.

Possibly: Keep track of overall sampling speed (over entire render) and calculate how much longer it will take from that. If saving time is included in calculating sample speed, the ETA will be more accurate. Or just use the last few minutes of SPS instead. (might be more accurate, but a little less precise)