That in fact is a bit of a surprise!
I am not surprised though, afterall I have never seen any official designation of J-20A anywhere. From the practice of older aircraft (not sure of J-10), the very first operational variant does not have any letter (A) such as J-7, the next upgrade was J-7I or (歼七甲). By that practice the initial variant should be J-20 (AL-31 or WS-10), the next with WS-15 will be J-20A(歼二十甲). However if AL-31 and WS-10 equiped are counted as different variants, we should have J-20 (AL-31), J-20A(WS-10) and J-20B(WS-15). This is what is assumed so far.
But, the plan was that WS-10 is for initial small scale delivery for operation, that is J-20 without letter suffix. The insertion of AL-31 was not part of the plan, not used in big numbers anyway, so it does not deserve a separate designation. Therefor J-20's first upgrade version (A) is WS-15.
What surprises me is J-10. We have seen J-10B painted on the front landing gear door, so it is assumed that the early variant with intake ramp and splitter plate is J-10A and there is no J-10 without letter. That is a practice different from all others.