Just remember that this only applies if your mash water/grain ratio is 1.6qts water/1lb grain.
Your boil-off rate is important and will probably be most affected by the diameter of the pot you boil in. Atmospheric conditions, the BTU output of your burner, and your methodology (like, do you pre-heat your mash water while sparging?) will also apply. With 9 gallons of collected wort (i'm assuming your target was 6), that's actually not too far off. A two hour boil would have taken that down pretty close to 6.
Also, don't shy too far away from overshooting your collected wort volume. It's always better to overshoot your pre-boil volume and have a longer boil than to undershoot your volume and have either a too-high gravity or (god fobid) have to add water at the end.