My Nike SDM which has been calibrated and has proved pretty darn accurate in races up to half-marathon (although less accurate perhaps than a GPS) recorded over 29 miles from start to finish on Sunday!!! That's about 11% out :-0 It's never been so way out before; in fact it normally undersestimates a bit.
I reckon all the weaving and zig-zagging that most of us have to do must be a factor. Maybe interference from other peoples' HRMs, timing chips, etc is the cause. And since it's not possible for most of us to follow the blue line all the way round, so the distance is bound to be a bit more than 26.2 - but could it really be as much as 3 miles more?!