Combining milk.

So I read that is fine to combine milk into the same bottle from different pumps and store in the fridge. But you're supposed to cool the milk before adding it to already refrigerated milk. Of course it's only good as long as the older st drop of milk in the bottle. Why do you have to make sure it's cooled before combining? Doesn't make any sense to me. I feed my daughter pumped milk from avent bottles only, and they fit directly onto my spectra pump. Sometimes if I don't get much out in one pumping, I will be able to fit another whole session in the same bottle. So I can take the bottle out of the fridge and attach it to my pump and pump brand new milk into the same bottle. Why is this bad? It saves me from feeding her from 2 bottles for the same meal and it's less washing up