My groups have always played it as what makes sense for a push, just that. Otherwise it would be a slide.
If you push him to column L or K, that is in some way pushing him closer to you, or almost pulling. Even if it is moving farther away vertically, it is moving closer horizontally. Plus, the origination point is from the opposite direction. I might allow it had you moved to M10 rather than K10, but I've got to enforce differences between the different forced movement types and even if you argue that you can't bring logic into D&D, as the DM I am allowed to beg differently.
Honestly, it's stupid for a power to be able to push someone in the opposite direction that it logically should, ie: the point of origin.
But if you're that upset by it, and would rather him be moved than take extra damage, I can budge this time. Just be a little more careful and mindful of how you use these things in the future. I don't want to have to butt heads over this again. I don't need you guys Rules Lawyering this game. That's my job.
Anyway, this makes my job easier, since now that kobold is less damaged, and is now flanking Mytos. Thanks, Agita.