Math question: MIDI CC volumes <-> DB gain

I have two external MIDI gears connected to my Audio card inputs,
when raising volumes of all audio tracks I need also raising the CC MIDI volumes for the external routed tracks accordingly.

So I made some tests to check how CC changes affect real (DB) volumes
coming in the card input.

I found that doubling a CC 7 value ends up in a volume raise of 12dB,
while I expected 6dB

To be more precise, the exact formula is D = (40*Log (V2/V1)) where D is the dB increase
and V1, V2 initial and final value for the volume CC. The log is 10 base.

I thought the right theoretical expression would be D = (20*Log (V2/V1)),
but there's a 2 multiplier I just cannot figure out...

Please note that I have 3.5dB of preamplification, but this should sum up to the expected 6dB, ending in 9.5 dB, not 12 dB.

Somebody can help me finding what am I missing here?
Thanks
Mario