I don't care much for accuracy, climbing prowess or whether someone has done slightly more or less than what their watches claim…..but just thought I'd add that my Garmin regularly gives readings of between 800 and 900 meters of elevation gain for rides around 13 miles.
I've also seen people finish one of our tours feeling and looking like they have done double the mileage we actually covered.
So whether altimeters are dodgy or not, it sure can be hard (but very fun and rewarding) work 🙂
EDIT: we tend to take the easy route up if there is one! So if you are climbing the steepest options everytime I can easily see 4000 ft of climbing adding up in a very short time.