Consider an NFA for $\Sigma^*$, one which has $6$ states. Every state is an accepting state, and every state is reachable. The organisation of the transitions is irrelevant.
Now, there are 5 redundant (but reachable) states in this NFA.
The corresponding DFA will have just 1 state.
Regarding "Why would someone have an NFA with $5$ redundant states?", that is a different question, the answer to which doesn't affect the fact that there can be a DFA of $1$ state for an NFA of $6$ states.
Consider a person who is trying to convert a complicated regex into an NFA. It might not be obvious from the regex that it is reducible to $\Sigma^*$, and it might not be obvious from the NFA either. So, that makes a valid usecase for an NFA with $6$ states with majority of them being redundant.