The only way I can rationalize it is to figure that I've had more exposure to Spanish being spoken around me than French—a lot of my classmates growing up spoke Spanish exclusively until they were ten or eleven, and even then they spoke mostly Spanish around each other. If I have nothing else to do, I'll flip to the Spanish channels once in a while and watch a telenovela or a fútbol telecast; there just isn't quite the same amount of "resource" easily available to me in French. I treat Spanish as my second language and French as my third just in order of having been taught phrases and whatnot.
On the other hand, I've caught myself watching a quasi-documentary in French (the "Making Of" the musical Le Roi Soleil) and not knowing what was going on at all because I was listening purely to the sound as opposed to for words. It was a bit like how you read when you're tired—you read the words and don't put them together in your head to create any meaning. Similarly, I was listening to a particular song about an hour ago and thinking, "This Spanish sounds funny..." before I realized that it was French being spoken (sung, rather) in a heavy Spanish accent.
Has anyone else realized this? I'm assuming that I'm not at all unique in having this revelation.