strange transponder issue

[Geo]

Hello.
Last weekend I had my first long x-country, from Pennsylvania to Minnesota and back the next day.
On my way back I was on flight following and was talking to ATC.
I was at 7,500 feet, and the "pressure altitude" readout on my Garmin 327 was indeed indicating 7,500, (as well as both my Dynon D10 and my backup altimeter). But ATC told me that my mode-C readout was indicating 7,800 feet.
How is that? I thought that the readout on the pressure altitude window on the Garmin was the same that the unit was sending out.
Two more pieces of info: My encoder is an AT-150, and my pitot/static/transponder was just recently checked out (and passed) by an avionics shop here.
Anyone can explain the discrepancy?
Thanks

[647jc]

ATC or the tower will adjust your pressure altitude to true altitude based on the current altimeter barometric pressure setting at the tower. If you have set your altimeter to the same setting the tower is using then your true altitude should agree with what the tower tells you, more or less.

[vic syracuse]

The only time you will see your altitude readout on your Garmin match your indicated altitude is at a baro setting of 29.92. Actually, that is a good way to check it. So, unless you had 29.92 set in your altimeter, ATC is right in that your altitude was wrong.
Either something got calibrated wrong, or perhaps your altitude envoder drifted. I think you mentioned you have a Dynon. The D-10A had a serial encoder output that you can wire to the transponder. No need for your encoder.

Vic