I had exactly the same issue pricing a one way J flight from DXB-LHR. On the website it said 50k AVIOS plus £222.40 in tax. When I went through to the next page it said the taxes were £276.10 plus same AVIOS. The wording on the first page is clear that it includes everything.
BA GGL said the first page was an average, it wasn’t an average of anything, and then said they would send to IT to investigate. I agree it is dishonest and possibly false advertising - you have reminded me to chase them for an answer to my query.
Regards
aks120