I think the difference is it easy to spot the physically developed players that aren't going to do that much, whilst with the more technically gifted players it's harder as you don't know how they are going to physically develop. For example, Lukaku never looked to a special talent technically, whilst Rooney did. So it's not surprising that Lukaku doesn't look any better now they he did at 16 or 17. Whilst Rooney, though he might not have hit the heights hoped of him, he did go onto to become established at a top CL club and was twice YPOTY by the time he was 21.
It's a bit like Jack Rodwell in my opinion. Everyone blames his injuries but people forget there were loads of people questioning his actual ability before he got injured. As technically, he just was never that good.
For me, players like Cole, Rooney and Wilshire etc, should be judged differently to the likes of Lukaku, Richards and Rodwell etc. I see Oxford as being firmly in the second group.