trying to get to the bottom of it

So I went to go do a mile at the indoor track @ Udall,
Nike + recorded it as follows-
.79 of a mile-
total time 7min13sec-
@ a 9min 02 sec pace

so it's a .20 off ok

went to reid park and ran the dirt track, another mile
Nike + recorded it as follows
.87 of a mile-
total time 8min 7 sec-
@ a 9min 14 sec avg pace

ok so it was .13 off


Ran the cement loop at reid which is 2.99

The nike+ recorded it as follows

2.42 miles
total time-24min23 sec-
@ an average pace of 10min03sec

ok so it was almost half a mile off total

All righty so I recalibrated the + to the last reid park run, and set it to 2.99, considering I run on the outside of the pavement and in the grass it's probably 2.99.
I've heard it estimated @ 2.98.

I plan on running the loop again tonight @ 10pm