XP quantity based upon difficulty? I had not noticed before but I shall watch for it now.
I too noticed that focus affects difficulty. For example, if the peteran priest is well focused he seems to have more "green" conversion opportunities. For the few "yellow" conversion opportunities, a little time spent befriending the prospect seems to change the "yellow" conversion to "green".
Focus is critical to success in all endeavours, from catching fish and picking wildflowers to hunting a great bear. Though I find it interesting that some "yellow" interactions seem more likely to fail than others. For example, a yellow tag on "Hunt the great bear" rarely seems to fail with my heroes, while my poor spy has been sent to the stocks more times than I can count by having a yellow tag when she's trying to break someone out of the stocks.
Some actions don't have a consequence for failing, and you can try again. Picking wildflowers is one such action, and the good thing is that once you do succeed, you then get a nice buff that helps increase your focus, and thus your chances of succeeding at whatever it is you're trying to do.