Is there a difference between proportionality and direct proportionality?
Yes there is a difference. If two variables are proportional it just means that as one increases so does the other. Likewise if one decreases, so does the other variable decrease. Directly proportional means that as one variable increases the other increases proportionally to it, i.e. if one variable is doubled, the other variable is doubled. If one variable is halved, the other variable is halved.
