You are only off by 9%. Not a biggie. 9% won't change the final beer very much at all.
You likely just didn't get the same efficiency on your system that your recipe assumed. Next time, you could correct this by adding a bit more grain across the board, slightly finer crush, like the last poster said, or even just boil for 15-20 more minutes.
9% isn't a biggie, but you really should be shooting for the OG, NOT for the certain volume. You would have been right on your target OG for this batch if you had about 4.7-4.8 gallons as your final volume.
Oh, and sugars don't "burn off" during the boil. Only water, in the form of steam. Gravity (OG) is a measurement of liquid density, and OG goes UP the more sugars are dissolved in your water, so boiling longer reduces water volume without reducing sugar content, hence more density, hence higher gravity.