If x operations cost O(x) amortized then how much xy operations cost?
Amortized is not just probabilistic, it means that for some big enough $y$,  $xy$ operations can't take a long time and will guaranteed to be $O(x)$ in average in worst case (and therefore $O(xy)$ for all $xy$ operations), even through some of operations may take even $O(xy)$ time itself.  

https://stackoverflow.com/questions/200384/constant-amortized-time