The differneses are given below.

1. occurs 0:
   (It means it will create the default rows for the
internal table depend on the requirement)

2. Occurs 10.
   (It means  it will create the 11 rows for the internal table)

3.Occurs 100.
   (It means it will create the 101 rows for the internal table)

But where as in defauty the system is create the 8KB size of
the memory space for the internal table.

occurs 0:
          it allocates space for one more record if we need
occutrs 10:
          it allocates space for 10 more records after the 
records in internal table .

occurs 100:
           it allocates space for 100 more records after 
the records in internal table

if there is 101 records in an internal table  and u created
internal table with occurs 100 then it will insert first 100
records with probable space and remaining 1 record also
inserted with  again 100 records of space will be allocated.
so here 99 record of space will be waste. 
same to occuurs 10 also.

it there is more then 100 records in an internal table and u
created internal table with occurs 100, then 100 records
will be in abap run time memory and other records will be
put in other memory.

so it is used for memory allocation.