It would appear there is a volume error for one of these two books. One is from 2011 the other 2021 but both show volume, 4, and a common publication date.
I’m sorry but I’m not not sure what you’re saying.
I checked Core and the one from november 2011 is listed in Core as:
X-Men, Vol. 2 #20B
While the one from May 2021 is listed in Core as:
X-Men, Vol. 4 #20B
Screenshots of both after being updated from core. Went back and added the vol.2 one again from the series search and it came up correctly. Don’t know why the vol. 2 was showing the vol. 4 info.
That’s strange indeed. However you’re saying it is listed correctly now, right?
Yessir.