Yeah, this is one of those fun things the game doesn’t explain very well. Click here if you want to read the wall of text I wrote the last time people asked about it, I went into a bunch of details there. If you just want a short explanation, keep reading here.
The game calculates the decay time for your base from the number of interconnected pieces and their tier. If that time is longer than the server-wide maximum, it gets capped to that. Normally, the cap is at 168. The decay time for your base is 170, which then gets lowered to 168 because of the server settings.
However, during the summer holidays, the server cap is doubled. That doesn’t mean all decay times are doubled, just the server-wide maximum. Since your base’s decay time is 170, and that’s lower than 336 (168x2), you get to see 170 even though it was 168 before.
Does that help?