12 hour repeated timers sometimes have an hour missing

[birdman]

Hi, got a strange problem. At the start of the month I added 2 manual timers to record BBC Radio 2

And never have explained what you are actually trying to achieve.

Most of the time the recordings have been the correct length as they should. However 6 times in the past month I've had recordings that have been either:
10 hours 50 mins or 13 hours 10 mins long.

Why does this happen?

How are you setting the system time (Transponder or NTP)?
With these short/long recordings, did the next one start-up at the correct time, or follow on from the previous recording (in which case you'd probably have short/long or v.v. pairs).