I saw two 173 buses bunching up together at Bukit Batok sometimes, especially from Clementi direction, regardless during off peak or peak time. During off peak, the back bus tends to be empty without any passenger boarding.
To add-on:
I spotted 3 257 buses (2 VO2xs came first, then 1 almost empty SBS9132U) reaching Boon Lay Interchange at the same time.
Suspect the first bus lambat like crazy, 2nd bus is late by a few minutes whereas the third bus is probably earlier than the expected time schedule.