Just a hunch, but I believe the altitude display is taken from GPS and not pressure altitude (barometric pressure).
So given the town you live in
, the display is reading correctly according to GPS altitude which is a reference to sea level. Evansville is ~300m above sea level.
I believe you said the altitude was reading zero before. Is it capable of being "zero'd out"? Don't own this yet but I do plan on buying one. I just confirmed with a friend today that you can throw on another gps sensor (10hz) for higher accuracy.