You should not worry at all. If temp tablespace is configured as it should be it would yield simmilar picture as you are getting. In temp tablespace extents should not get deallocated after the sorting is done (so implicit or explicit coalescing on temp tablespace should never occure).

When sorting is over, the extents remain there and are available for subsequent sorts. Only when no "free" extents (that is, extents that are not in used by current sorts) are available will Oracle allocate next extents. So for example, if one of your create_a_realy_big_index commands allocate 2GB woth of temp extents, all those extents remai available for next sorts. Only when you create_an_even_bigger_index or perform tons of concurent sorts some new temp extents will get allocated there.

This saves the need for quite costy "allocate new extent" operation and dramaticaly increase the efficiency of sorts.