Any science behind this?

I've had techs say that the charge in my system (AC only, no heatpump) was A-OK. Then last year, I decided to have a full evacuation & recharge done. I can tell the return air is much colder now - the tech said the split was 17degrees, but we didn't measure before the evac.

Any reason why the temp. split is much more favorable, after an evacuation & recharge? Pressures measured OK before the evac.