recently I have noticed something strange which is not quite clear to me and I hope someone can explain this to me.

I am planing to use my Avios for the following flight:
CDG-DFW-CDG all legs are on AA
Dates: 4 Sep - 28 Sep

When I go to BA i get 50k Avios + 270 GBP (Tax)
When I do the same flight with the same dates on the same airline via Iberia.com
I get 60k Avios + 247GBP (tax)
however when I go to AA.com 60k AA miles + 51GBP (Tax)

Since is the same flight , the same airline and the same dates how come tax can be different ?? I have bad feelings that BA and IB have made this up ?