Let's see it now with batteries of the same brand.
My guess, this is just differing manufacturing methods.
2004 Honda Civic Hybrid "Eeyore" @ 49 mpg
-MaxIMA, OBDIIC&C, IMAC&C, Lower grill block
RIP 2000 Insight, 40k miles @ 69.2 mpg
2000 Insight, AKA "The Basket Case" or "The Go Pod" @185k miles @ 63.5 mpg
-IMAC&C, OBDIIC&C, Scott's aero panel, grid charger, cheapo sound system