Why did Time Machine have to re-backup System, Applications, Library folders?

I noticed that Time Machine was taking a particularly long time to perform a backup earlier today. After it finally finished, I used an app called BackupLoupe to check the size and contents, and it showed that my /System, /Applications, and /Library folders had all been completely re-backed up (but not /Users). I didn't do anything of note between the big backup and the one an hour earlier, so I'm really confused about why it needed to completely back up these folders when it should've just needed to incrementally back them up. I've learned a lot about how Time Machine works by reading some threads here, but I haven't found an answer to why this might have happened. Any ideas?