1099-R = 12,000, Box 15a 1040 = 12,000, IRS says that income was underreported at 10,500, Box 15b on 1040 was 10,500, where did the 10,500 come from I entered 12,000?

The letter states Shown on Return 10,500, the corrected amount 12,000.  This is the amount I put in 15a on return, 12,000.  The amount on line 15b was 10,500, was this a mistake by turbo tax, how was this number calculated?  Am I responsible for adjusted tax amount?