P-ATTI and ATTI are not the same thing.
If you set the switch to A you are in ATTI, and need the "multiple flight modes" switch to be on for it to allow you to use it.
When you have the switch in P mode (only active position if multiple flight modes is off) then you'll have P-something, with the "something" depending on the current sensor status. P-ATTI if nothing is available, P-OPTO if the OPTO sensor sees something useable, P-GPS if you've got GPS reception.
So:
You simply had no GPS reception where you are doing your setup, and now you somehow have some. You were likely in P-GPS when you were flying, just didn't notice it.