I have the same issue with keybr.com. Maybe it's because the tutors look extremely similar from my brief look.
In my test of fast and slow, where fast worked and slow didn't:
What was the same?
-Same browser
-Same computer
-Same operating system
-Same keyboard
-Same date, the tests back-to-back
-No errors
What was different?
-Typing speed
I suppose the cookies could have a ridiculously short expiry on them, though.