I believe the font used by the Keyboard Layout Editor is Engravers Gothic BT, whereas the font SP uses is called Gorton Modified, which is a modified form of the original engravers font used by Gorton for their machines.
The KLE one is merely an approximation of the SP one, since Gorton modified is not available anywhere as an electronic font.
Other similar fonts to Gorton Modified are Arial and Helvetica, but there are still small differences.
KLE has a number of things incorrect / inaccurate, particularly when it somes to SA sets:
1. The font, as mentioned above.
2. The colours. The correct colour is set to the keycap skirt / surround and the keytop is lightened, so they appear lighter than the real thing.
3. The SA profile shape is a bit incorrect. The actual keycaps are symmetrical, so the top is not supposed to be offset upwards, but rather central like the DSA caps.
In terms of testing layouts and getting a general idea of colour combinations it's pretty good, but we need a better tool for doing colour and profile-accurate mockups.
As a VERY rough idea I hacked together version of your mockup with slightly more accurate base colours (I'm not sure what legend colour you're using for the alphas). This is closer to what it will look like in real life if you use BED alphas and RCU mods.