oracle11g - Oracle Partitioned Table

i have partitioned table (mytable) on oracle (11g). quite large table, partitioned insert_date column (without time).

the problem that, count(*) gives incorrect result.

the query below returns: 5,726,829,673

select count(*) mytable insert_date >= to_date('01/01/2015', 'dd/mm/yyyy')

the query below returns: 13,076,228,720

select sum(1) mytable insert_date >= to_date('01/01/2015', 'dd/mm/yyyy')

how can possible? reason difference?

TY