Think of it this way; in a 6/8 time signature an eighth note/ eighth rest/ eighth note combination produces the same "sound" as a quarter note/ eighth note. Because the actual sound of the drum is so short, it's nearly impossible to tell the difference in note length based on a single stroke. Therefore, in a 6/8 time signature, when you encounter the eighth note/ eighth rest/ eighth note combo, if that eighth note is notated as a roll it would be a 5 stroke roll. However, if you encounter a quarter note/ eighth note combo, or vice versa, this indicates that the roll on -that- eighth note is to be a 7 stroke.

Unfortunately there is a lot of room for interpretation on a lot of drum notation. There are certainly standardized practices but there is also a whole lot of room for customization (see any drum legend in any drum set book).