Inasmuch as any standards exist between indexing systems, it's because smaller or lower-end manufacturers copied the market leader rather than trying to fight them.

There's often good reason for drift. For example, the cable pull of SIS shifters and derailleurs up through 10-speed road and 9-speed MTB was a relic of the 7-speed downtube shifters. When Shimano had the opportunity to do a ground-up redesign with their trigger-only Dyna-Sys / Shadow system, they chose more cable pull for lighter tension which is easier to shift and easier on the shifter internals. SRAM made a similar choice but it's not (directly) compatible.