Nice to "see" you Happy Face!
Making assumptions here, but a single tone control should only be controlling the filter "cutoff" point at which high frequencies are bled off to ground - after the two pickups are mixed together. That said, I wouldn't expect the tone control to account for any sudden jumps in volume.
The taper of the volume pots might be a factor(?) Typically audio taper pots are used for volume (usually designated with an "A" - as in A500K), as the change in resistance throughout their travel better aligns with how our ears respond to changes in volume. If linear pots were used there (these are sometimes designated with a "B" - as in B500K, or without any designation - just 500K) then there would be a jump in volume around the 8-10 position, and very negligible change below that.
Keep in mind thought that in any passive setup - whether it be vol/vol or a blend pot - the pickups are not electrically isolated from one another as they are being mixed, so adjustments to one will affect the other - and this will account for some of the wonkiness when blending the pickups. If an active circuit were used, you could individually buffer the pickup signals before mixing them to get around that issue.
I think I experience this in my basses too. Usually I get the "fullest" sound by having the neck at 10, and bridge pickup at around 8 - or off all-together.
