I realized this truth when doing a quilt a couple years ago. Then it occurred to me that the reason was that i start out tense when I'm doing FMQ, but after a while I relax.
So, I began warming up with practice sandwich quilts. I have some ugly fabrics and the same batting I always use (Warm and Natural) on hand made up for this purpose. About 10-20 minutes of warm up and playing with designs, and my shoulders and arms and hands have relaxed and my mind is engaged . Quilting the "real quilt" goes so much better after a warm-up.