There are probably a couple of reason that could explain it. The older ones have been growing in different conditions than the newer ones, such as soil and weather, that could explain the difference. It's also possible that the earlier ones were a different Morden rose. There are several different ones and a few of them are a brighter solid pink, Morden Belle, Morden Centennial and Morden Ruby.

Thanks Seil - I wasn't aware that Morden Blush might be named differently. I'll try to get a pic to post. It does look quite a bit like Centennial, though not bushy.