What exactly did you misunderstand about the kata?

100%

Do not use global variables. The initial value of result will not be 0 when your solution is invoked several times. Move result inside your function and use nonlocal in your nested function.

Nothing will prepare you better for a career in development than requirements like this :D

You haven't implemented this case:

• If two monomials have the same number of variables, they appears in lexicographic order, e.g.:
  "a+ca-ab" -> "a-ab+ac", "xzy+zby" ->"byz+xyz"

In the first example, 4a should come first before b since a < b in lexicographical order. Same with the second example, fk < kq.