That’s what happened during my 15k. The slowest downhill sections were also the longest downhill sections (which makes some sense, since those hills weren’t as steep). Instead of averaging 6:11 on those hills, when I take the length of the hills into account, I actually averaged 6:21! Put that together with my 6:31 pace for my uphills, account for the fact that I ran a longer distance downhill than uphill, and you arrive at my 6:25 average pace, which wasn’t fast enough to overcome my GPS error.
Head on over there to read the whole thing.