For August runkeeper says I ran 160.5 km, or just a smidgen over 100 miles. That includes warm-up and cool-down walks, of probably 500 meter each for each run. With 16 runs in August, that should mean around 144 km "real" running.
Garmin says my total is 138 (I never include warm-up / cool down on Garmin), which means runkeeper is only 6 km out, or 4.3%. That's far less inaccurate than I thought.
Numbers, metrics, statistics. A bit like lies, damned lies and statistics.