I understand that use of any shower or tub will get a Sim to full hygiene, but I assume the point value is based on speed that this happens. Otherwise, I don't know why there would be a difference in the values. All toilets, for example, are level 10 and all take the same amount of time for a Sim to use. If the hygiene scores are meaningless, then I don't know why they are listed.
Separately, the comfort / discomfort relief values for showers and tubs should be different (or at least utilized) since that seems to be an obvious value that is rarely used.