Different fields have different sizes, speed and publication culture. Thus, it is completely expected that h5-indices will vary a lot between fields. This effect is going to be more pronounced when larger values are considered.
Size: To push an h5-index from eg 199 to 200, the additional paper needs to be cited 200 times within 5 years. In a smaller field, this could easily mean getting citations from an absurdly high fraction of the new papers published.
Speed: The h5-index only counts citations in a 5-year window. In a fast moving field, that doesn't matter much. In a slower-moving field, this can easily ignore most of the potential citations a paper could get.
Publication culture: How big or small are the units people publish? How many references does a typical paper have? Do authors always explicitly cite the masters, or only if really needed?