I noticed the exact same mistake too.
Well for one, the ranking system is pretty bad, its all achievements that force you into playing completely unorthodox and not good. It makes you want to try, but in the end, achievements are a blight that forces people into hoarding it rather than playing it.
But in other cases however, i could definitely see if the other ranks also level up a little if you play engineer alot. Just enough to show that you are experienced in the game, and not just the class.