Coffee master ™ here
I’m not 100% sure so don’t quote me on this
My guess is like you said that even though they are both single origin they are coming from different growing regions. The soil, altitude, rainfall and temperature can all affect the flavor of the cherries. Generally speaking a higher sunny and warm elevation will make the cherries flowery and sweet. My guess to why casi is a seasonal is there is a specific spot for those cherries to grow where Antigua has multiple growing regions allowing it to be year round.
That being said Guatemala Antigua has more of a spice and cocoa favor to it where as casi has a lemon sweet and cocoa flavor to it.
Personally I really like Guatemala Antigua as a hot pour over and casi as an iced pour over. Because of their different flavor notes and my preferences I like coffees with spice to be hot and coffees with floral and sweetness to be cold, I feel like the cold brings out the floral flavors the best and makes the coffee almost taste like a tea than coffee. Definitely recommend doing casi as an iced pour over and if you don’t know how to do one check out the iPad>store resources.
I will check out my coffee passport and the coffee fact sheets to fact check and will edit if needed:)

Casi cielo is once a year release. Different varietals I think most likely.

One of the corporate supply partners used to come in our store all the time and get Guatemala and said it’s the most similar to Casi but that casi was a little more special. What she meant by that I don’t know but she was high up in the company so I just said okay. Before I ever worked at Starbucks casi was my favorite seasonal I didn’t realize it was so popular until after though.