My best bet is still that you are only a factor of 25.4 off. The rest is probably rounding errors, or different family keycaps have different widths. Or there is a non-25.4mm inch involved somewhere...
Yeah I have sent an email to SP regarding actual widths on keycaps. Either way if I don't get a response back I have placed a bid on
this so when they come in the mail (if I win the auction) I will be able to measure them accurately myself.
Edit: I have received a response from Melissa at SP regarding key cap dimensions on some of the larger keys. The following is the response from her.
Key sizes are below for our DCS keycaps:
1.25 Space - 0.903" wide x 0.715"
1.5 space - 1.090" wide x 0.715"
1.75 space - 1.278" wide x 0.715"
2 space - 1.465" wide x 0.715"
2.25 space - 1.653" wide x 0.715"
2.75 space - 2.028" wide x 0.715"