I think what EvilAngel is saying is ultimately, your sample size is 1. What you are perceiving to be a pattern is actually just the randomness of the system. Unless you have over 1000 of a key, the drop rate of keys to boxes is random. I'm sure it has some predetermined ratio, (let's assume 4:1 which are COMPLETELY made up numbers)

4:1 means 80% chance box, 20% key. Even with those odds, you can still get runs of keys instead of boxes. Statistics says it's not likely, but PPS seems to exist in a world where statistical theory doesn't matter. The devs SWEAR that everything is random and there aren't any other behind the scenes factors that influence the streaks of whatever you happen to get a streak of (be it keys, or triple EXP, or triple Pero or anything else).

The Random Number Generator sometimes just likes a certain 'range.' and for no good reason, you get streaks of things. Still random, but it totally looks like a pattern to the casual observer. Hell, I would SWEAR there's some internal modulator that makes streaks of shit happen, but I get called a conspiracy theorist for that and Nutaku swears it ain't so.

So assuming Nutaku isn't lying, it's all just random chance.