Some basic dba_free_space stuff

Just some basic questions about dba_free_space. I have a tablespace, which in dba_free_space shows some "chunks" of free space.
I'm wondering about the size of the chunks. If I delete a row for example, That one
small chunk of space surely isn't freed - so how does Oracle decide that a "chunk"
of space is free, after a certain number of deletes, does it do it's own coalescing
for example? There's a 400K block of space in the below output for example.
How did a "chunk" that size get freed?

Output is

tablespace             file_id             block_id
bytes              blocks
INTFD2                  15                    27
14540800          1775
INTFD2                  15                8267
409600              50
INTFD2                  15                8537         34930688
4264
INTFD2                  15                4187
7372800            900