Centenarians of Mardia

But as you level, don't you get stronger and gain more experience per day?

Yes and No, that's on reason things like this have to be re-forecast regularly, because as circumstances change so will the future. Other than that, the data doesn't change that much day over day.

Take Arch Mages for example - A single level doesn't make us kill any faster. Even 5 levels won't give us an improvement of going from say 3 hits ot 2, or 2 to 1. it can take weeks of grinding to get a noticable improvement, so even though true on a fundamental level, the growth rate is slow enough not to be a major blur to the results.

While it's true that the most recent data is more indicative of what the future will be, using an entire month of data smoothes it out because you may not neccessarily be leveling every day anyhow.