In his case, we're talking about 2-3 kilos between his regular spring weight to his GT weight. That's not like some other riders who lose double that or more.
As to why he can't be closer to his ideal weight? Probably his metabolism is a bit different, and i also think a lot of it comes from the idea that he needs to be more heavy for classics and TT's (which he has proven numerous times is BS, since he dominated TT's while skinny, and became WCC and Olympic champion while skinny).
Let me jump back a bit here:
Like I posted above, RE has amazing palmaris, especially since he's only 25.
If he continues on the same path for five more years he will have an amazing career.
If he wants to compete with TP, JV, and whoever comes along next in
stage racing, he will need to live closer to his race weight for 12 months a year. We never really know what riders' body comp is, but at CdD he was viably leaner than he was a month ago. He needs to be training and racing very near his GT body comp sooner. You said that 'possibly his metabolism is a bit different',* even more important that he maintain a consistent body comp then so that his body knows how to metabolize energy most efficiently in that body scenario.
*if you want to get picky, everyone's metabolism is 'a bit different'.