I think you need to split the 2 aspects of a pellet flight.
The pellet is subject to a force in the barrel which reduces to zero when it leaves. The initial velocity is reduced in flight by air resistance (drag). So initial velocity, final velocity is a combination of distance covered and the fudge factor BC (which covers all sorts of things and is one of those "all other things being equal, BC is a constant" but perhaps not a constant).
Once airborne the pellet is subjected to a lateral force aka wind (no wind, no force) for the duration of the flight time. The force depends on profile, wind speed and direction (but lets say it is precisely perpendicular to the line between muzzle and target). Force is mass x acceleration. So if the net force is the same a lighter pellet will experience greater acceleration. The initial horizontal velocity is zero (say) so will speed up for the flight duration and the net effect is drift in the wind.
Therefore the critical factor is flight time.
For a lighter pellet flight time has to be shorter to have the same lateral displacement as a heavier pellet. The difference between initial velocity and final velocity has to also take in to account the vertical path i.e. a high lob versus a flatter trajectory which determines flight time.
So from my experience a 7.9gr Express doesn't take less wind than an 8.4gr Exact unless the velocity is much higher (and some rifles need a tweak to achieve this but obviously go over the 12ft.lb. if you then use 8.4gr)
So for a quick test - multiply muzzle FPS by 8.4 and 7.9 for each pellet and see if they are different e.g. if an Exact does 780 then an Express needs to do better than 830fps if all other things are equal - which they rarely are.
of course a twitch on release could impart an initial sideways velocity which adds to the wind and hence the "How did it go so far left when the wind is coming left to right? .. shouts of frustration"