As far as I understand, swarmia calculates the epic scope creep as the number of additional child tickets divided by the number of original child tickets. Feel free to comment & correct me!
number of additional child tickets
scope creep = --------------------------------------------
number of tickets originally in the epic
0% means no ticket were added during the epic.
100% means you doubled the amount of tickets during the epic.
Story points, time spent, etc. Anything that is not in the equation above. You need to be careful when interpreting scope creep. In the real world, the number of tickets might not always be as meaningful!