[Typo3] checkbox logic?! 10boxes in two rows

[T.Joga]

Hi! Whats the logic in the checkbox values?! There must be some, because 
it is working!

If box number 1 is selected value in DB is 1;
1. -> 1
2. -> 2
3. -> 4
4. -> 8
5. -> 16
6. -> 32
..
1,2 -> 3
1,2,3 -> 7
1,2,3,4 -> 15

ok, multi select values are counted 1+2+.. but how I should separate 
different values from each other?