(update: sorry, it seems I misread your description — too early in the morning... :)

but the occurrence rate for some months is actually 0, ...

When you have an input line with less than 5 months columns like

L122Q    97.56097561    100    100    100
[download]

how do you want to tell if it actually should have been

L122Q    0    97.56097561    100    100    100
[download]

or

L122Q    97.56097561    100    100    100    0
[download]

or

L122Q    97.56097561    0      100    100    100
[download]

i.e. how would you disambiguate?