I believe you're right. From my limited memory of that period, it was a mechanical constraint.

Keycaps tended to be molded with a hollow cylinder or stalk on their base, which fitted through a snug round aperture on the keyboard base and pressed against a spring or other restraint. Pressing the key down against the spring actuated a pcb-mounted push-switch (or bridged a pair of adjacent connectors on the pcb) that provided the keypress signal. Pressing a wide key off-centre would cause the plastic stalk to bind against the enclosing aperture. Forcing the user to press direcly above the stalk mitigates this - hence the raised part of the keycap.

There is a stack exchange question about this at [1].

As to why the shift keys were wider to begin with, I'm not sure. Perhaps a consequence of the lack of the mechanical constraints that forced typewriter keyboards into a strict grid due to the interleaving of the lever arms. Some keyboards, notably the Commodore PET, didn't use wide shift keys [2] though.

It is worth noting that keyboards in that era were machine-specific, and often hard-wired to the main system box. Afaik standardisation and interoperability didn't happen until RS232 and, later, ps2 keyboards were introduced.

[1] https://retrocomputing.stackexchange.com/questions/16471/why...

[2] and let me just say here that the PET keyboard was truly awful, even by 80s standards. Just shamefully terrible.

The keyboard on the Apricot uses round capacitive foam pads under the keys. This means the keys had to be square, or they needed a mechanical thing like the space button on the photo here: https://www.baffo71.com/details.php?id_img=7

So, I think it is a mechanical/electrical limitation.