DISCLAIMER:I think you're in a sparsely populated area so this might not work out for you, but if you have somebody nearby, local so to speak, that also has a horizontal beam, and maybe as well a vertical up and working like you, could you do a few simple signal tests with him, comparing each of your horizontal signals to your verticals and see if you can see a signal or audio difference?
First off, I am not a fan of trying to figure out how a antenna works, have no need to know, don't want to know. If the antenna does not work good, then I throw it away and buy another one! It's like the engine in my truck, have no need to know how it works, as long as it gets me were I want to go!
Ok, now as far as the I10K goes. I have notice that this antenna does pick up local signals that are horizontal some times fairly good. Mostly at a distance of 25-35 miles, but, only when the tx station has his beam pointing in my direction.
But also, these stations were running just 3 element Yagi's or 2 element V quads, so not much of a real comparison here. Location's make a big difference too.
So why does the 10K received the horizontal signal so good, I don't know, for this antenna has only been up for a couple years and prior to that....I've had no vertical antenna for the 30 yrs before that.