TW,
Well, it's a little bit of all that.
First, I like doing thing like this, and I also like challenges, when I saw .skin file and figured out I could make it, I tried, and here is the result. Screen size is also one of the reasons, and personal preference to horizontal versus vertical calculator design. There is one horizontally orientated skin, but it fills most of the screen estate in 1366x768.
Since I read about programmable calculators (some 30 years ago, when I was teenager) I always tried to sketch my own version, most of the time horizontally orientated, because, although vertical orientation is most suitable for 'in hand' usage, horizontal orientation is much better for 'on desk' usage. And most of the time those where 'ahead of time' designs (with to big screens and too small dimensions) :-)