The set rowcount option stops processing after a certain number of rows are returned to the client. With serial processing, the results are consistent in repeated executions. In serial mode, 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. When set rowcount is in effect, each row is written to the network buffer as it is found and the buffer is sent to the client when it is full, until the required number of rows have been returned. To get consistent results, you must either use a clause that performs a final sort step or run the query in serial mode.
