I think this is tied to the 90% effort model (which I thought was attributed to Carl Lewis, but I can't find a source). The A group are pushing themselves, but

I think this is tied to the 90% effort model (which I thought was attributed to Carl Lewis, but I can't find a source). The A group are pushing themselves, but they have a confidence of being at the top of the pack, and that slight bit of relaxation that lets their body flow freely through the water. The B group are striving and pushing and tense, putting 100% effort in all the time to try to get to the A group. This extra tension in their body is actually slowing them down. But from a mental state, it is also putting them in a position of excessive competitiveness. They are more focused on beating the other swimmers to get into the A group. The A group aren't trying to beat the other swimmers in the A group. They are trying to beat themselves. They want the other swimmers to be better, in order to push themselves to be better. The C group know they are in the bottom half, and have very little to gain from hyper-competitiveness. Might as well get along with everyone. The D group are just happy to be there.