Well, if you normalize the experience points by dividing the listed values with that received when your character is at the same level, the level difference modifier is the same for each opponent.

When your character is at a lower level (L) than the enemy (LE), the experience bonus is given by (LE - L)/6 (capped at 0.5, so at 4+ levels below an opponent you get the same experience as 3 levels below). I guess it doesn't help to go after high level opponents too early...
The formula for the experience E given the base experience E' (when your character is at the same level as your opponent) is then:

E = E' (1 + (LE - L)/6 ) when LE-L = 1, 2, 3
E = 1.5 * E' when LE-L > 3

When your character level is more than 1 level above your opponent, the drop off in experience is in the shape of the inverse square of the level difference. A rough formula would be:

E = 3.3 * E' / (L - LE)^2 when L-LE = 2, 3, 4... 14

If I had time to compare the predicted experience points to the observed, maybe a pattern to the discrepancies would emerge, or an explanation for the constant, but for now that's pretty close.