RE: 7mm Mag ballisitcs?
Evidently the 150 grain is loaded to a higher velocity either by a heavier powder charge or with a different powder. Are you basing your findings on chronograph tests you performed yourself or a chart put out by Winchester?
If both the 140 grain and 150 grain are loaded with lets say X grains of ABC powder, the 140 will always have a higher initial velocity out of the same rifle or test barrel. It's simple physics. Of course bullet design could cause effect things at longer ranges. A flat nosed bullet may shed velocity quicker than a HPBT due to air resistance.