Could someone explain?

The one in the front page doesn't work in BTS

Could you be a little more specific?

I can say that Combat is decided by the Strength of the two combating units. Their are many factors that can play into this. For example:

Warrior attacks and archer on a hill.

The Warrior with 2 Str has Combat I which provides a 10% boost.
The Archer with 3 Str has no Promotions (Upgrades) but the Hill and Unique Archer ability provide a total of 50%.

This leads to the warrior having 2.1 str and the archer having 4.5 str. Thus the archer has more than double the chance of winning as the warrior does.

My example is rather basic but I'm sure someone else will give a better description addressing your problem.

Assuming he meant how combat outcome is decided once the strengths have been calculated.

A strength 2 unit vs a strength 1 unit does not have a 2 to 1 chance of winning (66.67 %) as far as I've observed anyways. And I dunno how the calculation is done either. But thankfully you know see the odds in game before you attack. Don't think game difficulty have anything to say here at least (Thankfully)