Yes the taxable amount of SS can be up to 85% on 6b. Is line 6a the amount from your SSA-1099 box 5? It's very common for the IRS to change some other income which will make more of your SS taxable. Check all the lines on your return.
Maybe you have the wrong amount on line 4b or 5b for 1099R income. And make sure the 1099R amounts are on the right line 4 (for IRA) or 5 (pensions and 401k). If it is on the wrong line the IRS will miss it and say you didn't report it and add it in which will make more of your SS taxable.
