The x220 keyboard is possibly the same keyboard as in the T520 series. I don't notice that much of a difference between my x220 keyboard and the T520 keyboard, but in the x220 it even felt sturdier which means: better.
It is (the same). It might be a different FRU (since each keyboard manufacturer almost always gets their own FRU), but it's the same mechanical design.
The difference is probably due to support: the T520 has more internal space due to its size and cooling requirements, and while it's certainly quite good at supporting the keyboard, the X220's smaller size and very, very high component density means that there is very little free space beneath the keyboard.
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Current laptop: Lenovo ThinkPad T420 (running Debian Wheezy) - kestrel [pending retirement]
Current workstation: IBM Intellistation 9228 (running FreeBSD 9.1) - blackbird