Stfflspl mistyped the percentage to number equivalence in an earlier post. 70% =0.7 not 0.07. But if you use the % values in your formula (rather than converting them), you shouldn't have any problem.
I am guessing that you did not use the ROUND function in the formula in column AA. If you did not, while the percentage values in column AA are displayed rounded to the nearest percent, the actual values in the cells are not rounded. You may see "20%" in column AA but the true value may actually be anywhere from 19.50% to 20.4999...%. In the IFS formula, the first will will give the answer of 1, the second will give 2, even though the input looked like 20% for both. You will have to fix that for your IFS formula to work correctly. Using your description of the cells as "Cell one" and "Cell two" the formula in column AA will be
=ROUND(Cell two/Cell one,2)
With that fixed, your IFS formula is
=IFS(AA14<20%,1,AA14<40%,2,AA14<60%,3,AA14<80%,4,AA14>=80%,5)
You did not specify what to do if AA14<1 so I assumed anything less than 20% will give the result of 1
You did not specify what to do if AA14>100 so I assumed the result will be 5