Twins in the NICU

My twins were delivered via c section due to twin to twin transfusion at 28+4. They're doing well in the NICU. I know I have to use this time to recover. I have my husband, my parents and siblings helping with my recovery so I'll be fine. My concern is my babies nurses are saying they won't be released until their due date ( June 15th) and I know they have to gain weight, be able to breathe on their own, etc but what's is the likelihood of them actually staying until June? I pump breastmilk night and day. I just don't see the logic behind keeping them longer than mid May if they are doing so well.