Question

explain why the conversion ratios used between the SI and imperial systems are not nice, round numbers like those used to convert within either system

Answer 1

Because within each system, units are defined in terms of others in the same system using nice round numbers, but no unit in either system is defined in terms of a unit in the other system. The one exception is the unit of time, and there, the relationship is a nice round number: 1 metric second = 1 imperial second.