No, not possible for differences to cancel. The bitwise OR of a bunch of things is zero only if all of the individual things are zero. The difference between two things is zero only if they're equal. Therefore, the final value of x is zero only if all the elements of MAC_computed and MAC_received match.

(And -- the main point here -- how long it takes to do the calculation should be totally independent of what's actually in the arrays: no early termination, no operations that take different amounts of time depending on how many elements match, etc.)