To be a little more precise. Even bulk record upserts are limited to the number of records that can change at a time. There is a QB Help article on limitations I recommend referencing, use the help to look it up. Because of this limitation, there will be differences in timestamp data on larger data sets.
Acknowledged. To your point, I have noticed with a larger set of records that there might be a slight difference in seconds for some records when looking at the date modified. It's still awful close and good enough for me for now.
