The creature is also treated as one level lower for the purpose of level-dependent variables (such as spellcasting) for each negative level possessed.
What does this exactly mean?
by creatures (i.e., monsters) there is usually no level. Does this apply to hit-dice-dependent variables as well?
With variables it is meant:
things that are computed during an action (at the moment)
things that were computed using a level
I ask since all the other effects as the ability checks and attack roll penalties are affecting values that are depending on the level but are not really variables (as the range of a spell computed with the level)
An example: casting enervation on a dragon does affect it's spell resistance value?
the SR value of the dragon is dependent on it's age/hit dices but is not based on its level
the SR is not really a variable