What I HATE is the 'hybrid' philosophy. ERRRRRR.. be either Metric or imperial NOT both.
Frankly it should be 100% Metric, there's no good reason to be hybrid.
You have introduced a new term to the discussion... Imperial... To be honest, I have not heard the term before, except for the way liquids were sold in some countries back in the 1960's. i.e. Imperial Quart. I remember Narragansett beer's GIQ or giant imperial quart, which contained 20% more than the U.S. standard quart. It was the beer makers way of getting around the legal limitations of a beer bottle having no more than quart of beer. It was very popular with college students in the Northeast.
Getting back to the bolts, we have what was 3 standards in the US, SAE (society of automotive engineers), which is the coarse thread. USS (United States Standard), which is fine threads, and Metric, which measures the thread pitch between the peaks of two threads in millimeters.
I tried looking up Imperial measurement, and found conflicting information, but what I was able to deduced from what I found, it is the measurement of inches and feet in the US, and that Imperial is the generic term for both fine and coarse bolts in the US. Learn something new every day.