I can't say how it ended up that way in the first place, but I think at this point it's just a matter of compatability with previous designs.

The 18F4520 is based on the 18F452 who's pinout was made to be compatible with previous 44pin devices such as the 16F877.

Within each group, there are many similarities.
ie. Many 18-pin devices share the same pin configurations as well.

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