Yes, plot-aris seems to be right ! After many experiences on this fact during my works, the zoom is completely dependant on where is the cursor !
When you're too far from your model and you point on a empty space with the cursor, the zoom is too slow...point on a part of your model and is getting faster.
When you're closer to your model, if you point on a empty space with the cursor, the zoom goes too fast and the "camera" goes behind your object...still pointing on your model and the zoom will be slower until the camera goes behind it !
But that's right, it's a little boring sometimes when you're working on small parts of a big model, or in wireframe mode !!