the difference is a softer spring inside the normal followers have fairly strong springs in, so if you get valve bounce the lifters take up the extra space right away. anti pump ones with softer springs are much slower to react so dont take up the extra space (not in the amount of time you would sensibly be in that situation at least! )
thats how i understand it anyway!
remember kids, always change cams and followers together, if you want the parts to last longer than a few k anyway! ( 4k im my case ) ah well important lesson learned!