It looks like if X days and 0..24 hours have passed since an event, Bitbucket will display this event as "X days ago". Although this is logically consistent, consider the following scenario with three Pull Requests:
This screenshot was taken on Monday at 11:00. All these PRs were last updated on the same day (Friday), but the first two after 11:00 and the last one before 11:00. This difference of a few hours translates into a displayed difference of a whole day, creating the illusion that the PRs were updated on separate days.
IMO after 24 hours have passed the exact hour of the event is not displayed anyways, so it should be ignored from the calculation altogether, and only the day part should be taken into account, which would result in the (for me expected) result where all three PRs are displayed as "last updated 3 days ago".
Of course, the "real best" solution would be to implement the ability to disable relative dates.