Keywords and Parameters
Specify in bytes the size of the object's first extent. Oracle allocates space for this extent when you create the schema object. Use K or M to specify this size in kilobytes or megabytes.
The default value is the size of 5 data blocks. The minimum value is the size of 2 data blocks for nonbitmapped segments or 3 data blocks for bitmapped segments, plus one data block for each free list group you specify. The maximum value depends on your operating system. Oracle rounds values up to the next multiple of the data block size for values less than 5 data blocks, and rounds up to the next multiple of 5 data blocks for values greater than 5 data blocks.
Why do you expect only 1 block upon creation? Where do you think that the Master Free List will be placed? Oracle needs a separate block for it (exactly after the segment header, if you are curious where it is placed).
If you run OPS, the situation is a bit different if you use BLOCKING FACTOR in the init.ora parameter GC_FILES_TO_LOCKS.