When I was first learning to type Hangul, I quickly found a lot of logic in the 두벌식 layout. Consonants on the left, jamo that can be doubled in the top row, nasals in the middle, aspirated in the bottom, vowels on the right, ㅏ ㅑ pairs above each other, ㅗ above ㅜ like arrows, and so on.
What has been always bothering me and making my life a bit difficult is that the order of the aspirated consonants is different from the unaspirated, without any obvious reason. For example, the layout of QWERTY is structured as to prevent from arm jamming on a mechanical typewriter. The 두벌식 layout seems to be built on easy memorization and some logical patterns, however I wasn't able to find much on the 두벌식 layout in English.
My first idea is that it is because of ergonomics – that would make the more common letters right to left, with the most common letter under the index finger. Unfortunately, I wasn't able to find a frequency list of individual jamo. I coded up this JSFiddle, that counts the jamo frequency in any given text. After feeding it a few Wikipedia articles (mostly about countries, which may have caused some bias, especially with 국), I arrived at a result with about 100,000 letters total (full result in percentages on pastebin). The order of the letters from most frequent to least frequent is not matching with the hypothesis: ㅊ ㅌ ㅍ ㅋ. Maybe more and better data would show a different result? (You can try the tool with more text, or if you have some long text or a Korean language corpus, you can send it to me.) Either way, the aspirated letters are not even very frequent in the language, the benefit would not be huge. However the ordering of the upper row seemed to match, if we assume the data was skewed by the 국 in countries name mentioned earlier.
My second idea was that it is because of alphabetical ordering, but the current order in South Korea is ㅊㅋㅌㅍ.
Any other ideas as to why the change in the order? Why not just leave it as it is in the upper column for consistency and clarity, since the rest of keyboard seems to be made for easy memorization?