My social security disability benefits were reduced two years in a row from same tax return information. Isn't it only a year?

I took my pension age 60 while on disability..the end of that year and my benefit was reduced from information on that 2013 tax return...I had reduced benefit for all of 2015 to be re evaluated in Nov 2015..today I got it and it is reduced even more because 2013 was the last year I filed and they used same information to reduce my benefit..I didn't have to file for 2014 nor will I for 2015.. Since I had no other income for these two years or did not have to file why are they still using 2013 tax inf to determine my benefit?