Math girls go through school with the necessary math calculation skills but without a solid approach for applying those skills. This mismatch leads to a lack of confidence with math in new situations, even with the same preparation as someone else.
Math problem solving is how to creatively apply familiar calculations to new situations.
A parent told me about this years ago when I used to attend a math circle with my eldest. At that time, my eldest was very strong with calculations but relatively weak with problem solving. These were the books that she used with her daughter (who is now a math major).