I'm claiming a moral victory on the grounds that I have nicer legs than Mr Scott.
Sorry to be a bit blunt, because it's unfair on you, but the problem isn't the distance you ran - that was 10km as measured by the Jones Counter and recorded on the certificate of accuracy. The problem is your Garmin's measurement of that distance. We all covered 10km, and speed is distance over time. Distance was 10km, regardless of the inaccurate measurement provided by your watch, and time was 58:51.
GPS is inaccurate and inconsistent, which is why it isn't allowed as a measure of licenced courses at any level of competition.
www.runnerslife.co.uk has some good blogs on, from top runners. It was reading the training that the guys do that made me realise they aren't superhuman, just hard workers, and encouraged me to actually go training properly at the turn of the year.
Of the blogs on there, most are good to read to some extent. I usually enjoy Matt Barnes and Aaron Scott though, because I can relate to the distances they run. I think the key is to find someone who writes nicely and runs similar races to you - so for me that means an ultramarathoner's blog is just words, but a top 10k runner has a lot more resonance,
The course has been measured with a Jones Counter and has a certificate of accuracy, so Garmin readings are irrelevant. For what it's worth, mine also came up long, but then it often does because it's a stab in the dark at best... I think at one point it had me running in the hedge.
I'm fairly sure chip time can only be from when you cross the mat, because that's when the chip activates 'you' in the computer. The race positions will be by gun time of course - if you look on Power of Ten there are time lags of up to 20 seconds between the two times displayed, which suggests the chip time isn't from when the gun went.