Somehow, going through it in my head 10 times, I kept coming up with new reasons that's only 20 years. "1985, 1995, 2015, that's 20.." I finally had to check w

Somehow, going through it in my head 10 times, I kept coming up with new reasons that's only 20 years. "1985, 1995, 2015, that's 20.." I finally had to check with a calculator and that's when I realized I skipped 2005. This same kind of thing happened to me on that clock problem. Why does this happen?