I'm a little confused over the disguise skill and the rules for perception. Although the general state when someone is merely disguised but not as a specific individual is explained (opposed roll) when it comes to modifiers etc there is a different emphasis.
The disguise skill states you cannot disguise yourself as a specific individual. However under the rules for using perception to pierce a disguise the modifiers are all based upon how well the individual knows the person that you are trying to disguise yourself as.
From Disguise
"Disguises are general - you cannot disguise yourself as a specific person"
From Perception (pierce disguise)
"If the disguised creature is impersonating a particular individual that you know, the GM may grant you a bonus to your perception check"
"The DC for perception checks to pierce a disguise are adjusted according to how familiar the observer is with the particular individual the disguised creature is masquerading as,"
The only interpretation I can come up with is that the section on pierce disguise is written for when a character has an ability that specifically lets them disguise themselves as an individua, although I'm struggling to find such an ability.
Cheers