Wow, that's a depressing calculator!
It is impossible to retire before 55 with that damn thing starting in your 20s. Staring at age 22 with no savings, a $50k salary, and maxing the savings at 25% you can't retire until 58! If you only save 10% you can't retire until 71 which includes social security.
There is some seriously constrained thinking about the concept of retirement baked into that calculator. The MMM 60-70% savings rate, <30% income replacement, and 10 year career would probably make their heads explode. They are assuming a nominal return of 6% and 2.3% inflation for a 3.7% average real return. Thank goodness US stocks actually average 6-7% real over long time periods!
I understand the audience is the average Joe who thinks saving 5% is great and you have to retire at 67 because that is when you get SS, but the pessimism on display here is really extreme.