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Basically all taxa of everything are arbitrary. The only exception I can think of are when there are 2 genera in a family, 2 families in an order, etc... then you can be pretty sure they think it's a split point in the evolutionary history of a taxon. Otherwise, you've got say 15 species in a family... are they 3 genera each with 5 species, 5 genera each with 3 species, or wat?
There are guidelines, but people argue about the guidelines. So you have arguments about genera, families, orders, classes, all the way up the tree. This is especially true in the plant kingdom where species are so numerous and closely-related species can be so different from each other.
To finally address OP's question, I don't know the specific family you're talking about, but it's not surprising that two different websites have two different taxonomies. Probably the two websites come from different academic traditions of how to classify.
