Elliptial confusion

I meet a friend a the YMCA every morning to "work out" for 30 minutes.   

We really often chat more than we sweat, but we like to give it a noble sounding name anyway.

This fall, however, we decided to get "serious" (notice the quotation marks again) and try to burn 300 calories every morning.  

So we've figured out we can do that in 30 minutes -- and sometimes less, on the elliptical machines. 

Sadly, we've also learned that there are certain machines that we can do that on faster than others.

I'm not sure if we're cheating, or if the machines are cheating, but something fishy is going on at the Y and I'm a part of it.