Anatomy scan

I had my anatomy scan this week and I am now 21 weeks pregnant with identical twin boys. My doctor called and said that twin b had bilateral flexed wrists and they are sending me to maternal fetal medicine. During the ultrasound the technician mentioned he could not get a good image of his arms because he was basically sitting on them. Has anyone experienced this? It sounds like it could just be baby’s positioning, or it could be something like club hand? Everything else is normal. It seems weird to me that one of them could have an issue like this and the other wouldn’t since they are identical