Not sure what one row at a time means (or indeed what the relevance of a batch is) here; the specification of T-SQL makes it quite clear that the statement will update all the rows that satisfy the where condition unless prevented/modifed so that only one row is changed by one of two things:

(a) set rowcount = 1

(b) a trigger (either an instead of trigger that handles only one of the rows in the inserted/deleted tables, or an after trigger that restores the original value of all but one of the rows affected).

I imagine you would have noticed if either of those applied, so maybe one row at a time means something else?