This is very true (and valid for most sport fans, think tennis).If we mean by this question who most overachieved their physical talents, we are mostly talking about the riders who had the greatest mental talents.
There’s something a bit strange about the way in which cycling fans tend to value the physical capacities of riders over their mental ones.
Expectations over a rider's future are generally based on his physical attributes: endurance, recovery, power output, speed, etc. His potential assessed accordingly.
There's little room for analysis over someone's mental strength, which includes a number of aspects: drive, ability to suffer, composure, decision making, etc. There's no data available, no numbers to crunch. It is virtually impossible to assess if a junior has the mental strength to become a consistent pro, unless you know him personally.
Take Gianni Moscon for instance. Solid career among the u23, goes pro with Sky in 2016 and has a very promising first year.
Then in 2017 he finishes top5 in both Roubaix and Lombardia. The common view among cycling fans at the end of the year is that he might be the next rider to win all 5 Monuments. He's 23 after all.
But he hasn't finished top10 in another Monument since and the biggest win in his palmares is arguably the Tour of Guanxi.
Is he an underachiever? Probably yes, if you only consider his physical attributes. Does he have everything it takes to be a top rider at WT level though? I doubt it.