Optimum Level of Deceptive Attack

[kure]

I DO like stuff like this, but i can see some flaws on this table.

if I have skill like 20, why ever should I get only deceptive attack 1?

result will be Attack 18 and defense penalty -1

when all rolls 17+ is fail anyway, it have no benefit to have net skill 18.

If your initial odds of a landed blow (attack succeeds, defense fails) are 50% [Skill 15 against Defense 10]

is it?
defense roll 10- alone have 50:50 or 50% chance,
but there is also chance of failing 15- attack roll which affect result.