Personally I read this as being a cumulative cap, not an increasing one.

The weekly cap of 450 stays but unearned tomes can roll over into future weeks. E.g. if you only earn 300 tomes this week then next week you can earn 600 because you still have 150 outstanding. The total number of tomes you can earn at any given point in the patch cycle remains the same, but you're not "punished" for starting late or not capping every single week.