The extra 10 minutes is to make up for the fact that life doesn't always run smoothly. Your bus might run late, the bus you're connecting onto might run late, the traffic might be bad, your watch might be slow, all sorts of reasons. So, just to be on the safe side, TfL's software permits an interval of 70 minutes rather than an hour for that free second journey. That's very kind of them.
Previously I assumed that if you caught a bus at 10:30, you had to catch the second by 11:30 to get it for free. But no, it turns out you'll still get it free all the way up to 11:40.
Thanks to clever wording, what it says on the TfL website is technically true.
But in fact you have 70 minutes, thanks to TfL's grace period, which might be an unexpected treat... or might even stop you running madly down the street to catch your next bus.
Even better, when the Hopper is extended next year to cover unlimited changes, the 'free hour' should still stretch to 70 minutes. As many buses as you like, in 17% more time than you thought you had, sounds like an absolute bargain.