It looks like something wrong with some SI stations. Weren't synchronized?
We synchronize before the season, the batteries are low on some of them. Which ones were bad?
Today I ran from S to #41 and difference between Garmin and SI was 29 sec (4.01 vs 4.30). From #77 to F (2.46 vs 2.59). Last Sunday almost all SI stations in Green Course showed wrong time.
that sounds like a reasonable drift for units with old batteries over two months -- #41, and start and finish too. I think I'll be changing batteries this winter if all goes well.