Isildur: Why are some alphabetization heading letters grouped together in pairs? (2022-07-22 22:05) [#12108]

There are four sections that for some reason pair consecutive English/Latin letters:

E (え), F (ふ)

P (ぱ行), R (ら行)

U (う), W (わ)

Y (や行), Z (ざずぜぞ)

Is there any particular logic for this, or is this just a holdover of something like the letters previously having been grouped in bigger ranges like A-H, I-O, P-R, S-Z, and one-by-one the letters were broken off the ranges and moved out to their own sections, as the list grew longer with new additions? At this point, it's a little confusing that there are these pairs; it seems to me the list should be broken up entirely by letter.