Your water level in your tank is set by the overflow (or standpipe - depending on your setup), & as water is lost from the main tank, some will be drawn up from the sump to replace it. This keeps the level in the tank constant & is a main advantage in a sump/wet-dry setup.
Actually, "water dropping 2 inches in the sump" does not tell me how much water you are losing. If you have a big sump, 2" is a lot of water. But if you have a tall narrow sump, 2" might just be a few gals - a reasonable evaporation loss.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.