Me, a few buddies, and the gf all have have the same Ski Tracks app. I have an iPhone 5. One of my friends has an android phone and the gf has an iPhone 4. Whenever we all start our apps at the same time and ride together all day and then check our stats at the end of the day we all get different results.
The gf's iPhone 4 is the worst offender. It consistently reads faster and steeper than anyone else's stats. There are days when we all cruise at the same speed all day and ride the same runs then when we check the stats, the gf's phone usually records speeds 5-15 mph faster than all the other phones. One day she had a maximum slope angle of 54 degrees when I'm sure we didn't exceed 30.
I take these ski tracker app stats with a grain of salt. Whenever I achieve a new top speed I'm always skeptical.