If I had non-deductable contributions to a separate IRA prior to 2016 what impact does the 2017 distribution I took from another IRA which never had non-deduct contrib?

On my 1040 line 15b my distribution from the IRA  was reduced by $3800 as compared to line 15a is this correct? What should it be, same as 15a? Why? I am over age 65.