Remember, what is easier and makes more sense is not necessarily the same as what the rules say. I agree that lances should only get 1-for-1 on Power Attack when used one-handed while mounted. However, I also agree with Hyp that the rules as written imply that lances get 2-for-1 on Power Attack regardless of whether they are used one-handed or two-handed.
The rules as written have nothing to do with designer intent or game balance or logic or what is suitable for your campaign. The fact that the designers intended something different, or game balance goes out the window, or an illogical situation results, or you dislike the implications of the rule for your campaign all do nothing to change the fact that the rules say what the rules say.
Of course, if all the above are true, it's a pretty good indication that the rule ought to be changed, and house-ruled until it is changed. But until it is changed, the rules as written are the rules as written.