It's not just a question of strength, but also style. Even against other human players, I encounter that. You get used to countering certain types of moves by certain players, so you're good against them, but then other players might give you problems.
For instance, there's one guy I play regularly at a local club who I always beat, even though his rating is slightly higher than mine. When we both play against other opponents, he's often surprised that I lose against an opponent that he wins against, because he thinks I'm better than he is. It's just that he can't handle my style of play (or I can handle his easily?), but his style of play might be better suited to playing against the guy whose style is tougher for me.
With computer opponents, this is often even more pronounced. They tend to be very strong tactically, but weaker in just about every other area, especially endgames. If you're good at endgames, and you avoid open, tactical games, then you're likely to do better against them than if you go into wild tactical positions, regardless of overall strength. That probably doesn't apply to the strongest settings of these programs, where you'd have to be a master (at least) to beat them, but I know in the older versions of ChessMaster that I've played, it definitely applied to the lower rated computer personalities.
I do think you should practice more against humans than computers. If you don't have time to go to the local club, at least play online against humans at FICS (freechess.org) or something. That will give you more realistic games for preparing to play against people than just practicing against computers.
--Fromper