The Federal Review shows capital gains of $17,831. The brokerage 1099B shows capital gains of $8,914.09. Why the discrepancy?

You may have other sources of income that are taxed at capital gains rates that may not be listed on your 1099-B.

For example, if you have a 1099-DIV form, there may be an amount listed in box 2a for Total capital gain distributions.  

If you have any pass-through income being reported on a Schedule K-1, these forms may also have capital gains that will be reported on the federal review. For example, this 1099-K for a partnership would show long-term capital gains on line 9a.

I recommend reviewing your source documents to confirm the amounts reported on the federal review include all reportable sources of capital gains income.