Oracle8 and above rounds off extents to a multiple of 5 blocks when
more than 5 blocks are requested. If one requests 16K or 2 blocks
(assuming a 8K block size), Oracle doesn't round it up to 5 blocks, but
it allocates 2 blocks or 16K as requested. If one asks for 8 blocks,
Oracle will round it up to 10 blocks.
Space allocation also depends upon the size of contiguous free space
available. If one asks for 8 blocks and Oracle finds a contiguous free
space that is exactly 8 blocks, it would give it you. If it were 9 blocks,
Oracle would also give it to you. Clearly Oracle doesn't always round
extents to a multiple of 5 blocks.
The exception to this rule is locally managed tablespaces. If a
tablespace is created with local extent management and the extent
size is 64K, then Oracle allocates 64K or 8 blocks assuming 8K-block
size. Oracle doesn't round it up to the multiple of 5 when a tablespace
is locally managed.