If they matched each other exactly for 3k but by 10k were 500m apart that suggests something else might be going on. If the problem were nothing more than your wheel circumference being out then by 3k I'd expect them to be 150m apart.

If you've experienced a bit of GPS drift that could potentially explain a 500m difference. If you rode through an area with dodgy GPS reception that kind of thing can happen. I've seen a couple of places where roads go through cuttings and every single time, whichever GPS I'm using, my GPS shows me drifting about 100 yards off the road as I go into the cutting before snapping back to the road as I emerge from it. There's a clear view of the sky, so I reckon the rock face reflects a strong enough GPS signal to confuse the processor. Whatever is going on, a situation like that adding a 100-yard drift off-road and then a 100-yard drift back onto the road immediately puts circa 200 metres of extra distance onto the log. If you've been through tree cover or a tunnel or some such that will do it as well.