Air Consistently Reads 500psi Low?

MK2i and T1.  Bought them in November, dove in Cozumel over Christmas with them.  I still have my analog SPG.  It appears the Garmin setup consistently reads 500psi low, regardless of using air or nitrox (and setting appropriately for each).  Very disconcerting to get the vibration that says I'm "critically low on air" or words to that effect when the SPG still shows enough to wind down the dive normally.  Full tanks read about 2500psi though they're filled to 3000+.  This was the same over 16 dives. 

I'm not sure if it's a T1 sensing/transmitting issue, or something on the MK2i "interpreting" correctly.  My dive shop is overhauling my regulator currently but once that's done we plan on trying some additional testing/troubleshooting with the T1 and MK2i together.  In the meantime, any thoughts, similar experiences, or suggestions?