Boil off rate depends on your system, how vigorous the boil, and the weather. Often the average is 1gal per hour. Its not too big of a deal deal because you can add top off water to hit your desired volume. If you use all the extract and hit your desired volume your OG will be as stated per the recipe.
As far as your fermentation:
1) how did you pitch the yeast?
2) at what temp did you pitch?
3) how much yeast did you pitch?
4) what kind of fermenter are you using?