This is a very fair point. It sounds as though the OP is describing a transient peak - these can be caused by either a GPS/software glitch spiking your data, or (possibly the OP's case) when a transient speed reading is missed by the data sampling (especially if you have smart recording set). After using a 305 for getting on for 2yrs I've stopped worrying about anomalous spikes/peaks and focus more on the sustained efforts.

OP have you tried uploading to diffent software packages? I like to use Sporttracks on my PC, and IMO Strava is the best online GPS app. Maybe see if you get better numbers through one of thoses? (both free to try)