Hi Guy`s ,
It really is there , it is not off by much , but the C-6 is very slightly lower than the rest of the other white keys.
I have seen this on the following:
PSR-2000 / 1000
PSR-2100
PSR-3000/1500
PSR-S900 / S700
The Tyros of course it not affected by this flaw because it has a different key-bed ( FSX ).
I have seen this problem first hand buy owning some of the above ( PSR-2000 ) and I have seen it on a number of keyboards listed above in various music stores , both new and used.
I always take a look whenever I see on of the above , and sure enough C-6 is always slightly lower than the rest.
Most of you know me and I would not make something like this up !!

The reason most of the owners don`t see it is because it is really very slight and does not effect the playing aspect of the KB , it is strictly a cosmetic issue.
I thank you all for looking and if you are not bothered by it then that is great , or maybe it IS just the ones that I have seen or owned ( enter "X-Files" music >LOL )
Take care,

Gary