Find the dissimilar number among the choices:
$$9271,7633,6724,5745$$

Let's analyze the properties of the digits for each option:
* $9271⟹9+1=10\text{ (1st + 4th digit)}$, $2+7=9\text{ (2nd + 3rd digit)}$
* $7633⟹7+3=10\text{ (1st + 4th digit)}$, $6+3=9\text{ (2nd + 3rd digit)}$
* $6724⟹6+4=10\text{ (1st + 4th digit)}$, $7+2=9\text{ (2nd + 3rd digit)}$
* $5745⟹5+5=10\text{ (1st + 4th digit)}$, $7+4=11\text{ (2nd + 3rd digit)}$

As observed, for $9271$, $7633$, and $6724$, the sum of the middle two digits is constantly $9$. However, for $5745$, the sum of the middle digits is $11$. Therefore, $5745$ is the dissimilar number.