Confirmed by our printing partner, they consistently use A7 size envelopes (5.25 x 7.25). However they could in fact come from different envelope vendors/sources (based on availability, price, terms, etc). SO although they are all A7 size envelopes there can be some variance in the exact dimensions. You'll notice a difference in the flap size/style as well.
Based on your feedback we have stressed to them that consistency is important however we can not dictate their sources to remain identical.
As for your sleeves, I'm sorry

The best I can share is to use a sleeve that will accommodate the size variances in an A7. I guess kind of like having your skinny day jeans and not so skinny day jeans. I know it's not ideal but something we have to be prepared for.