Of course, you're assuming that the BC for both pellets is the quoted average BC for that pellet in your gun.
There are around 50 different die's of JSB Exact 8.4's in 3 different quoted head size - some perform much better than others, I don't know how many die's of 10.3 or how many head sizes they come in?
For an example - I have a batch of 8.4's that give a BC of 0.0265 in my rifle at 50m (measured with a chrono)
Try your comparison again, but this time - compare an 8.4 pellet with better than average BC against an 10.3 with worse than average BC.
Of course, if you can find a batch of 10.3's that give a better than their quoted average BC in your barrel and they group well and remain stable in flight out to 50m then I think you're onto something.
But that might be part of the reason - it's easier to find a better than average 8.4 than it is to find a better than average 10.3
There's an added complication for spring guns too - the fine balance between slam/bounce and pellet weight/fit