My understanding of it is that the 65535 number has always been the most experience the score screen will show, however it wasn't until a few months that Cooky reported a bug that people were obtaining more experience then that from base boosting, they were getting around 100000 exp per match but the score screen only showed 65535. So staff changed it so there is now a max, which is 65535, to avoid overpowered base boosting and to make it so the score screen wont have inaccuracies with the experience.