lets say I have to take 10 steps ,

if I know the answer for # of ways to take 9 Steps , let say it's n1 - I just need to add one more steps to n1+1

you're counting steps to 10, not how many distinct ways to 10.

Generally from n-1 to n, we only can do 1 way, no other choice

also from n-2 to n, we only can two 2 steps, otherwise it's duplicated