anyone one know why of the 7 AD's tried (8 including the one implemented at the start) there are the different answers? 0.4999999963909431 vs 0.4999999963909432

anyone one know why of the 7 AD's tried (8 including the one implemented at the start) there are the different answers? 0.4999999963909431 vs 0.4999999963909432 vs 0.4999999963909433 I assume if is some kind of IEEE math thing. given that IEEE allow `(a+b)+c != a + (b + c)` but where is it occuring exactly?