Thanks legacy, I had my colleagues investigate: you had one subsonic instance accumulating $4.04 worth of usage and one semisonic instance for $2.15 run during the month. Neither of those instances reached the spending limit for the month. Previous month charges on a semisonic instance maxed out correctly at $4.50.
When you calculate your budget, keep in mind that the top spending are per-instance. If you start a new instance during the month, that will start a new counter for that instance. Even if you kill an instance and start a new one, with the same size, the counter resets.