Teemo Support detected at adc for rating?

So played teemo support to try and get level 7 champion mastery, I think the system may have thought my cs was too high for a support and calculated my result as an ADC, kind of annoying as my score was pretty good, second most dmg on team etc. and achieved an A-? Maybe I don't understand the system properly, idk. any confirmation or ideas as to why I may have received this grade?. Thanks fams!