I Ran With A Garmin 305 Forerunner, Would A Hill Make A Difference In The Distance?

I ran a repeat in 6:20, it was either a 5:17 pace or a 4:52 pace, a big difference, according to yahoo and google maps it was 1.3 miles, but according to the garmin, which is unaccurate in elevation, it was 1.2. Any help would be useful?