How does the unlock credit system work? I thought that you got 1 credit each time you ranked up, but this is not the case. So how do I earn maximum unlock credits?

It is usually every other rank up that you will acquire 1 credit. It is not always every other rank either. So it is not every rank up, which you have already come to realize.

Ok thanks, guess I will just have to play the heck out of it to get all the unlocks :)

Just get to 25 and you'll have enough for all unlocks (except for findallfive ones)

"It is usually every other rank up that you will acquire 1 credit. It is not always every other rank either. So it is not every rank up, which you have already come to realize.
"

Great, we established it was fucking that 8 and a half months ago.