How many weeks are there in a One Piece year?
I'm going to go out on a limb, and suggest that "once a week" is not precise. Most people would find it more convenient than "once every 7.6 days" or worse "every 1.1 weeks".

While it would be interesting to have complicated shenanigans, and I have no definitive proof that there aren't any, I think it's more just quick communication.