Froome was lacking in the Dauphiné and not as strong in the Tour as previously, so he had a better buildup for a double.
That sounds pretty speculative to me. The bottom line is that when a rider attempts a GT double, just that alone mostly overwhelms anything else he does. Roglic has had a very impressive year, and I don't have a problem with anyone being suspicious about someone coming so close to winning the Tour, then a month later being a major contender in the Vuelta. But I don't see that this is unprecedented. In addition to Froome's several Tour-Vuelta years, both he and Dumo came close to a Giro-Tour double.. That is three months at peak condition.