Yes, I've noticed that as well, but I love that! I use my 15-91 to piece quilts and sew thick leather. It always amazes me that the tension knob never needs moving. The leather, when putting 2 pieces together, was a full 1/4" thick! I used a heavy nylon upholstery thread. When I finished the upholstery project, I went right back to piecing. I just love my 15-91.
I have a 201 as well, and I go back and forth between the 2 when I'm quilting.