Time Machine Quirk?

Normally when running a backup, the Pull-down Menu AND the Time Machine Preferences show the number of MB (or GB) that have been backed up out of the total number to be backed up. However, maybe since that upgrade to Catalina, now I see, when the number of MB (or GB) to be backed up is reached, the number of MB (or GB) backed up KEEPS COUNTING.

For example, today the backups counted from KB all the way up to the total to be backed up of 535MB. However, when the count showed 535MB of 535MB, it continued to count beyond the 535MB until it got all the way up to 561MB and then simply said, 561MB backed up. Then it went to the Clean-Up.

Does Time Machine reevaluate the Back-up to be done during the Back-up and just continue to copy data?

What's the story here?