Okay ... four things effect changes to your ratings.

1. The result (obvious) ... a win will generally increase your rating (it won't go down), a draw (ratings generally get closer, the higher dropping and the lower increasing), and loses decrease your rating.
2. Your rating before the result was achieved (again obvious)
3. The difference between the players (again, reasonably obvious, not sure though what happens when a player is not rated, sorry). Where the difference is great and the higher player loses both change dramatically (big penalty on the higher rated player and big reward for the lower rated player) or if the higher player wins both change only a little.
4. The number of games which have been counted to achieve that rating (not so obvious). There is an "averaging" which is based over all the games played, if there are a lot of games the change will be minimised reflecting the establishment of an average position. When a player has only played a few games, there are less games to average against so the changes are more significant.

I believe that it is this last component which caused the your opponent's rating to change but did not effect your rating.

Hope this helps ... for more info check out the FAQ