Exactly as jraskell said....
The default values for the library are 544 ms for min, and 2400 ms for max...think of these almost as the trim values you would have to adjust if you were using the servo with say a standard RC radio, so as to limit it's movement when you are setting up something..
you can control these settings as well, but issuing a command like this as well:
servo.attach (yourPIN , min , max) so something like servo.attach(9, 520 , 2000) for example, to trim the min and max
and then specify the values that will make the servo not slam into it's extremes, and maybe stop twitching...