Interesting observation. Of course we need to consider the positioning of the actual viewing area, for example my MX6000 on a motor stand has the lower edge of the screen at ~50cm from the floor.
Personally, I think I would rather look down than up, and perhaps have the eyes of typical dialogue participants level with mine, so the reference for the eye height measurement could be perhaps 1/3 down the screen. 4:3 vs. 16:9 is an issue as well.
But I suspect all this variation is just due to how the overall design will look, as the bigger screens would sit awfully low using this reference... even looking at the older models, the bigger LX might probably look a bit silly with a stand that brought it to the same screen level as the MX.