Honestly, some people hate Barca more, some people hate Chelsea more. Some people loathe Madrid or Leeds, and, of the thousands of registered caftards, one or two might have a personal loathing of Plymouth. Everyone has their own personal preferences and circumstances, has come under different influences, sat in the pub next to different people who supported different teams and gave us different reasons to hate or quite like those same teams. Why do people get so worked up about other people's loyalties in football? What difference does it make?
This is especially true of a globally supported team like ours. Some people tap into the history and local rivalries. Some people are more interested in their own location and who their own friends support. Horses for courses, we're all United fans.