Useless size guide

Good day, Bought items online on the singles promotion i.e. buy 1 get1 free, on purchasing the items the size chart of both items reflect Medium is Sizes 12 -14. I receive the order the one item is size Medium UK 10 and the other one is Medium UK 14. Emailed online support, am told I cannot exchange the free item if I want to return I can return both items or sell the bigger size item. Who is at fault here when the size chart is generic for all items. Am told the garments are different makes; if that is the case ADIDAS size guide must speak to each item. With this kind of non existing customer service and not taking accountability, will not be making future purchases from ADIDAS.