Lately, a lot of blocks with detailed models (hoppers, etc.) have been getting detailed collision to better match the models, and the same goes for the bell, except the standing variant. Its collision box is a singular cuboid, whereas the other bell placement types have much more detailed hitboxes consisting of several cuboids.  This can be easily demonstrated, as you can walk up the front of a bell placed between two blocks, but not up the face of a standing bell, even though they appear to be similar enough for the trick to work.