deladion yep, I think it's to do with where the waistband starts. On my maternity trousers as they're low cut and start the elastic under the bump, it creates an all in one smooth bump effect and you can quite clearly see it (esp in the evening as I am bloating like a balloon by the end of the day!)
I bought a jumper on the weekend which was fantastic bargain, and I figured just looked like a large oversize woolly (the sort you'd wear with leggings / skinnies etc) and I could wear it now and afterwards and no-one would be any the wiser - cue the OH saying "jesus, you can definitely tell you're pregnant in that!" so maybe I'm wrong!
Normal trousers etc don't have the same waistband so it sort of cuts in half and I look a bit tubby as I always have.
I also think maternity clothes enhance the bump with the patterns and cuts (aside from the extra room..) I've found a lot of stuff is stripes etc which just draws the eye