An issue with GPS accuracy

Hi,

I have an iphone 6, my g/f has an iphone 5. We're both up to date with the latest IOS and both using the same mobile network provider. We love walking and use Endomondo so that we know how far we go. At the start of the walk, we both start the program and I slip my phone into my back pocket and she slips her phone into my other back pocket (apparently women don't have back pockets, joke).

So why is it that at the end of the walk my phone says we've gone 10k while hers says we've gone 12k?

I appreciate GPS isn't highly accurate but surely something must be badly wrong for it to be this much out? I've checked the location settings on both phones and they're identical.

Anyone have any ideas what could be wrong?