"select count(*)" and "select single *" returns different result

Good day!

product version SAP ECC 6.0

oracle10

data transfers from external oracle db into customer tables using direct oracle db link

sometimes I get case with different results from 2 statements

*mytable has 10 rows

*1st statement

data: cnt type I value 0.

select count( * ) into cnt from mytable WHERE myfield_0 = 123 and myfield_1 = '123'.

*cnt returns 10 - correct

*2nd statement

select single * from  mytable WHERE myfield_0 = 123 and myfield_1 = '123'.

*sy-dbcnt returns 0

*sy-subrc returns 4 - incorrect, 10 rows are "invisible"

but

1. se16 shows correct row number

2. I update just one row from "invisible" rows using se16 and 2nd statement returns correct result after that

can not understand why

thank you in advance.