actually, I think there is a deeper reason why the diff holes on the rear tray... if that was the reason they could made both speakers holes 6x9 or 5x8, and there wasn't dual speaker option anyway, or there wasn't a reason why make the blower hole on the same 6x9 speaker pattern. If you note the bower assembly, it FITS on the 5x8 hole the same, with the propper bracket made for the size
I'd say the reason is more "acoustic". You get left channel on front 4x10. When single rear speaker, the right channel is 5x8... both cone surfaces are MORE LESS THE SAME, able to reproduce lower frequencies.
when getting dual speaker, you get both left channels on the 4x10 and 5x8, and ONLY the right channel on the 6x9. More cone surface
there is a norm on audio, than 2 speakers are more less same efficiency than one 1.4 times bigger. So 2 10" speakers are more less the same than 1 14" speaker ( if they exist ).
Of course the 6x9 speaker is not exact to the 1.4 rule, but better than nothing, is what market offers