The amount of heat and absence or presence of rain/watering can make a difference in the color and appearance of some roses, and there may be other factors at work in the soil that we don't even know about. China and tea roses are variable at best, and their modern offspring can be too. After a while it's just something you take for granted and don't even wonder about.

I agree these roses are very variable in color. It is really interesting. It makes identification hard, though. The roses I was more familiar with before Teas and Chinas were so consistent in color. This has surprised me too.