I think she meant that if you chose to use a different increase, it will mess up the stitch count according to the pattern... like this:
k2, m1, k21, m1, k3, m1, k21, m1, k1. (52 sts)
If you changed a m1 to kfb, the pattern would need to be changed like this:
k1, kfb, k20, kfb, k2, kfb, k20, kfb, k1.
Since a m1 doesn't use a stitch to increase, and kfb does, you would knit 1 less before each increase.