If your SS numbers come from the annual statement, it assumes that you're working up to age 62 at your current income level. Neither of those assumptions may be correct.
And if you have military wages between the 1960s-2000, your SS contributions were "offset" by an additional $1200 year credit. That's not reflected in the SS statement, either, and it makes your actual SS check a little higher.
The SS website has an online calculator that lets you enter all of your income and you can boost that by the credit.
I haven't looked at it in a while, but it used to be SS's knowledge base ID #454 at http://ssa-custhelp.ssa.gov/cgi-bin/...F9jYXRfbHZsMT1
. But I'd be surprised if that URL works anymore.