The set rowcount option stops processing from continuing after a certain number of rows are returned to the client. With serial processing, the results are consistent in repeated executions as long as the query plans are the same. In serial mode, given the same query plan, the same rows are returned in the same order for a given rowcount value, because a single process reads the data pages in the same order every time. With parallel queries, the order of the results and the set of rows returned can differ, because worker processes may access pages sooner or later than other processes. To get consistent results, either use a clause that performs a final sort step, or run the query in serial.
