That is the difference between Permutation and combination.
In permutation order matters and in combination order doesn't matter.
CATI and TACI will not be same in case of permutation.
Suppose you have 1 apple and 1 mango and we have to select two fruits then there is only one way i.e. pick mango and apple (or apple and mango.) (this is combination). Arrangement doesn't matter since our goal is to just pick the two fruits.
But if we have to 1 apple and 1 mango ,then we can arrange them in 2 ways.
first apple then mango.
first mango then apple.
(this permutation). Arrangement matters.