How far can you trust technology?

I wasn't able to run my first 20 miler of this training programme at the weekend, like most of my fellow VLM marathoners. I was too busy celebrating my friend's birthday being pampered at spas, sporting Audrey Hepburn facemasks and eating cooked breakfasts to bother with all that long slow running malarkey.
Luckily this meant I got to head out on sunny Monday morning before I started work (late, of course) to take on the first 20 milestone of 2013.
Regular readers will remember I've only recently given in and purchased a Garmin, so this was also an inaugural 20 mile run for my new gadget.
I headed out on my tried and tested 20 miler - down to Richmond Park, one lap round and home again. This route has been measured on several occasions by Endomondo, with no deviations. It was a gorgeous day, if a little bit heavy on the large deer dangerously close to my running path, but I'm developing good 'don't scare the deer' strategies and managed to make it round the park unscathed physically. Mentally, however, it was a bit different.
As I tackled the gentle climb back up to Richmond gate, feeling happy about being on the last leg, I realised that my Garmin said 12.6 miles. My whole route is based on the fact that it's 6 miles to the park, 8 miles round and 6 miles home. But according to Garmin, I had over 7 miles left to run.
My first thought was, 'well, I'll have to add another mile loop in somewhere along the line'. I then mentally went through my route database in my head, working out which would be the least soul-destroying extra mile at the end of a long run. Once I'd remapped my run, I then realised something much more shocking. If my Garmin was right, I'd been UNDERTRAINING FOR MY LAST 3 MARATHONS!
I can safely say this is not something you want running through your head at this stage of your long run. Was Endomondo's GPS wrong, consistently, several times over 18 months? Was my Garmin wrong today? Had they moved Richmond Gate?* What had gone wrong? [AUTHORS NOTE: resist temptation to make pun about 'no eye deer' here]
I still don't know where the gap is, until I try that route again with the Garmin. Needless to say, I added an extra loop in to bring my run up to 20 miles, so I've either run the correct mileage or I've done over 21 miles. Neither of which will be a bad thing, in the overall scheme of things. But it's certainly taught me not to rely solely on technology for route mapping in future.

*(hey, I was tired and high on SIS gels, we're all allowed to have conspiracy theories in the last few miles of a long slow run)