I'm not aware of any actual standard for key spacing on the bottom row.
IBM Model M layout became an industry standard in the 1980s. It had a couple of 1.5u modifiers with an 1u gap (there's a fun story about that) on each side of the huge spacebar. Some vendors modified it a bit and put 1u keys in the gaps—typically to place backslash somewhere, because they were still shipping a big-ass/AT Enter key instead of what would later become ANSI or ISO Enter key (introduced by IBM too).
Then Microsoft Windows 95 came out, and Microsoft pushed the Start/Windows and Menu keys between Ctrl and Alt. It became a major selling point at the time. Some manufacturers stuck with 1.5u modifiers and tucked 1u keys between them, while others switched to uniform 1.25u key spacing.
Fast forward another 15 years, the enthusiast community started to grow and organize group buys for keycaps. The group buys targeted keyboard layouts, that were popular in the community at the time, i.e., Filco Majestouch (and other Costar boards) first and foremost—Leopold was second, but used a different stabilizer spacing for the spacebar, which was a big deal at the time. Those keyboards had 1.25u bottom-row keys.
Meanwhile, there were keyboards with 1.5u and 1u bottom-row keys too. Mostly Razer BlackWidow and other keyboards made by iOne at the time. They were abysmal quality, though, hence less popular, hence not a group buy target. Gaming-branded vendors have used manufacturers such as iOne a lot, because they could produce cheaper keyboards in larger volumes; in addition, there were problems with supporting in-switch LEDs with Costar stabilizers (which wasn't the case with Cherry's or iOne's type), and gaming keyboards had to be backlit, right?
Note that the tooling for those keyboards had already been in use for many years.
All in all, the "standard" bottom row is only a community standard based on what keyboards were popular at some point. It's not a formal standard, and the community is still a negligible market for companies that operate on a much larger scale.
