I was taught that the horses backside story was bollocks anyway.
That the gauge of these things was just a trial and error initially.
The wider you make it the more stable it is but the harder it is to get things to change direction.
Over the years that width was found to be the most useful at achieving carts and wagons that don't fall over in a gust of wind but also you can get them to turn corners as well.
Hundreds of years later we found out the engineering principles and maths to prove what wagon makers had known for hundreds if not thousands of years before.
People have tried to reinvent the wheel when it comes to that gauge and it never works quite the way they intended.
There is a whole heap of geometric ratios out there that humans have used since before the pyramids were chucked up for designing things.
Cultures which there is no proof even knew about each other or even knew there was such a thing as a horses backside where using the same ratios and gauges for transport sledges and things like that.