Were those your first cartridge changes? The level reporting I got from my initial set of carts was definitely strange. They showed full for a long time, then very suddenly starting moving rapidly towards empty. All subsequent carts have seemed more or less predictable.
I have rarely had more than two carts in warning mode at the same time. If you're getting strange behavior I would recommend weighing the maintenance tank occasionally, just for accounting purposes. And record the ink use data in the "Accounting" screen once in a while, I believe that to be accurate as to total ink usage, although maybe not for cartridge levels. Does ink seep when the printer is off? I don't think so but anything is possible. I have not seen any suspicious ink wastage, but I usually print every day with no long inactive spells.
The only big jump I ever saw on the maintenance tank was when I changed the right print head, that took it immediately down to 60% after being at 100% for a few weeks, but I don't recall seeing any outrageous changes in cart levels except a 20% jump on the photo black that I rarely use.
I have once in a while seen sudden jumps in the cartridge level graphs, but at times when I was already wondering why the cart was still so full after a lot of printing. For instance a 700ml Yellow went when from warning to replacement in less than 40 square feet, which maybe shouldn't have surprised me since Yellow is the most used color. I have never taken a cart out and found that it was not really empty, however.
Canon needs to improve cartridge level reporting. I know perfectly well from things like the "Accounting" data that the printer knows perfectly well how much ink it is using. Are they trying to hide something from us? I don't think so, but I would like to be kept more in the loop on that stuff, or at least read a helpful FAQ somewhere about it. Maybe I'll start writing down the Accounting ink levels whenever I change a cart.
But if this is the worst it's gonna do, count me happy.