When gaining the upper hand position, getting to the horizontal usually works to your advantage. Dropping your opponents into the horizontal plane in effect closes them in the up-down dimension. This makes it much more difficult for them to project force against you and is getting the upper hand into position to attack. From the lower hand horizontal position, it is tougher to achieve the spacing necessary to strike in. To strike from the lower hand position, the elbow extends and the shoulder flexes to straighten the arm. If this starts from the horizontal, the point of contact quickly rolls to the outside the sphere of defense of the upper hand defender. In effect, it's not possible to just slip through when the upper hand has established horizontal control.
While the upper hand position wants to close to at least the horizontal, the lower hand position has better positioning when the point of contact is opened above horizontal. When above the horizontal, the lower hand has the elbow below the wrist. In this configuration, it is possible (assuming the positioning is not too low) to either attack from the point of contact or invade the sphere of defense to establish the proper spacing to attack. With the wrist above the elbow, the straightening of the arm has a chance of reaching the upper hand defender. If the lower hand position is open enough, the upper hand defender does not re-establish defense on the point of contact (roll the attacker to outside the sphere) before the attack penetrates.
A similar analysis can be applied in the left-right dimensions with the body line. Closing to inside the body line or opening to outside the body line establishes offensive and defensive positions in a similar fashion to opening and closing to the horizontal. So, what do these thought experiments tell us? They illustrate that there is sound reasoning behind the first two of the four strategies. From the outside, close; from the inside open. The upper hand (outside position) should be closing in and down to cut off attacks and establish offense. Likewise, the lower hand (inside position) should be opening out and up to maintain defense and establish angles of attack.