OOP: random prefect question

[Julia]

No: HPFGUIDX 61932

It says in book five that prefects can't take points, that's why 
Ernie throws a fit at Draco before he finds out about Umbridge and 
her little wanna-be Spanish Inquisition.  But then why can Percy 
take points from Ron in book two (he takes five points I think)?  
You know, when they come out of Moaning Myrtle's bathroom?  Where 
was Moaning Myrtle this year anyway?

Julia, who dislikes inconsistencies, even trivial ones