It took me few days to notice it and just updated my
U9BL-S Review accordingly.
Maybe that's what others have described as "keys not aligned". I call it the "stair" effect. It makes me hit keys by mistake during game play on the numeric pad. Not too much of a problem during text typing. But in some games, I use the right pinky for the NUM0 and NUM1 and I kept hitting NUM1 for no reasons. Now I know why…
See the pictures of a U9BL-S with "stair" keys vs a Deck with leveled keys.
U9BL-S "Stair" Keys

Deck leveled keys

I'm not a fast typist but I suspect this might affect a fast typist who's not lifting his fingers as high as I do to move around from one key to another. He/she will probably hit the edge of other keys in his/her way to the next key to punch.
The above assumption will need to be confirmed by a pro :-)