If you think about how a RHP works, this makes perfect sense. They work by heating the surface of everything within their line of sight. The effect of this heats the surrounding environment and, as a by-product of doing this, the air around those objects. It only makes sense that the air temp would be slightly cooler than the surface temps when using a RHP. Since snakes lay on the surface, those are the temps you should have dialed in most accurately. The ambient air temps can go down to the mid-70°F range and you'd still be good as long as the surface temps are where you need them to be.
In summary, I recommend turning down that RHP a few degrees. 90°F is about the hottest they need in order to be able to do their thing effectively.