Newton's Second Law

Newton's 2nd Law states that rate of change of momentum is directly proportional to the resultant force. F=dp/dt.
Doesn't resultant force equal to rate of change of momentum? Why is it directly proportional instead of equal as stated in the law?

Thanks so much!!!

Directly proportional means there is a constant relating the two quantities. In this case, that constant is just 1 which makes it equal