Why is are my 5 minute and 10 minute ratings so different?

I'm pretty stable (stagnant) around 700 on 5-minute games, and yet over 1000 in the 10-minute games. I often feel the opponents aren't as good in the 10-minute games, yet our ratings are higher.

I guess the ratings are determined by a win/loss ratio or whatever, but was wondering if anyone else had the same experience or possible explanation?

Maybe not then.

So the longer the game, the higher everyone's rating is? 

Not always true. My bullet rating > blitz rating > rapid rating on this site.

But in general, it depends on your game play. If you use your time to ensure no pieces are blundered. longer controls suit your game play better (hence, a higher 10-minute rating than 5-minute rating).

Emm the rating scales are just not meant to be calibrated to each other. I think the blitz is lower for pretty much everyone below a certain point (like if your rapid is below idk 2000, your blitz is almost surely lower than rapid).