I got a NOS 5x8 speaker long time ago to fit in there because the regular aftermarket 5x8 speakers doesn't fit either on same screw locations. Need new holes. I didn't want to open new holes.
The only logic I can think off on the smaller size driver side is ( based on 3rd gens at least ):
on stereos setup with just one rear speaker the 5x8 allmost matches with the front 4x10 on cone speaker surface, so then the frequency reponse, being front the left and rear the right channel.
when you get dual rear speakers, we get front left, driver side rear left and passenger side 6x9 right channel.
Talking about audio, one speaker size being duplicated, gest the same response than one speaker 1.4 times bigger... let say two 10" gives teh same response than one 14" ( 10 * 1.4 = 14 )
based on that, a 6x9 speaker is aprox the same than two of the other ones...
thats the only I can think of