The
Scangauge uses the MAF signal for the MPG calculation. To prove it unplug your MAF and look at you instant MPG, it'll be really high. It also uses the IQ value, as you noted from your tune.
I would suspect a failing or low reading MAF or a bad connection. Some people fill the MAF connector with dielectric grease and that resolves the problem. But your car was apparently pulling smoothly so I don't know.
The timing value is relative to BTDC. A value of 3 is BTDC. Minus values are ATDC.