I didn't study too much meteorology in undergrad, but one thing impressed upon us is that any forecast beyond maybe 3 days is basically guesswork.

I think what might be getting observed here is that when forecasting that many days out, the local data becomes so unimportant to the model's outcome that the model is just reflecting historical climate trends. Which kind of makes both the same kind of model. Ie. when forecasting tomorrow, the current temperature and pressure data really makes a difference. But once pushed to 7 days, those data essentially become a proxy for typical weather at that time of year, possibly down-weighted by a lot.

I just woke up and I feel like I'm doing a very poor job trying to describe this.