Buddy Distances question

Is there an explanation somewhere as to what goes into determining the walking distances for Buddies? Some are obvious (Legendary's, most 10km egg's being 5km buddies) but there are quite a few that just don't make much sense or seem to be holdovers from when their eggs changed from one class to another. (like Eevee being a 5km egg now and 5km buddy when it used to be a 10km egg)

Like why is the Pikachu line 1km but the other three Starters 3kms? Why are so many of the 2km eggs (like Oddish, the Nidorans, Abra, Machop, etc.) 3km Buddies?

Is there some official explanation for it?

(and yes this is because I'm annoyed that I have to walk my Scyther as far as I walked my Larvitar in order to get the needed candies to power it up now that Scyther seems to have vanished again from my area after having about 10 nests clustered in an area I can usually get to, but was unable to do so during that particular migration due to being out of town for the 3 weekend/holiday days where I could normally go play and so I lost out on getting about 200 Scyther candies (or more, if I could Pinap them reliably) and now I need to walk a Scyther to get the candies I need to power them up and yes this is one long sentence shut up I'm emotionally ranting about something I shouldn't be so emotional about and yes I should just go get myself a coffee and a muffin and focus on something more important like kittens or puppies or how to prevent thermonuclear war with North Korea)