Problem with Vout of Honeywell ASDX Analog pressure sensor
Why do you expect the voltage to be exactly equal to 2.5V? From the datasheet:

http://www.farnell.com/datasheets/1850579.pdf


Given Pmin = 0 (since this is an absolute pressure sensor), Pmax = 30psi (since you are using a 030PAAA5 model), Vsupply = 0.5V and applied pressure = 14.7psi (760 mmhg approx.), we get Vout = 2.46 volts. For a slightly higher pressure of 770mmhg = 14.9psi you will get 2.47 volts out and for 811mmhg = 15.7psi approx. you will get 2.59 volts output.

If you want to measure not the absolute pressure, but the difference between the atmospheric pressure and some other input, either get a second sensor and subtract the output from both sensors from one another or get a gage sensor (XXXMGAA5 in your case).

By the way, you did not include the datasheet.