Running Gear: Garmin 305 accuracy

I've been running with the Forerunner 305 for about 8 mos, and have noticed discrepancies between it's distance measurement and the known length of the run. It's consistently overstating my distance. Some examples:

1) Every week I do track repeats. 800m is just under a half mile, so I shoulod be hitting .5 miles just after the completion of the 800. However, on my Garmin, the .5 mile mark occurs *before* I complete the 800, by about 50ish feet (I run in the inside lane the whole time). 

2) I just ran the Kaiser Half-Marathon in San Francisco, a certified course. My wife's 205 reported 13.1x miles. My 305 reported 13.26 miles. 

3) Same thing with a 5k a couple months back. 3.25 miles instead of 3.1x. 

 After the 5k, I thought maybe it might be a bug in the device software, so I used the Garmin Web Uploader to update it. Sure enough, my version was several months out of date, so I was optimistic. 

But the inaccuracy is still happening. Any ideas?