This is because Oracle allocate the work part by part to the parallel servers not at one stretch.

For example, a table with 1000 blocks and a query with 4 parallel servers, Oracle will divide the table into 4 parts i.e 250 blocks each. Out of 250 blocks first it allocate 100 blocks then another 100 then remaining 50. In this case you will see each parallel session 3 times.

Hope this clears.

Thanks,