Your "properly set up system" is apparently doing nothing other than running your single job. The vast majority of real-world systems have to deal with antagonists.

All of the characteristics you mention are true on production systems used in large scale fleets...and yet "bumps" happen...because there's never one thing happening. It's all the things, and it's always changing.

I'm gonna guess you do finance. A six microsecond fiber oneway is a thing of beauty. There are certain technical luxuries associated with that domain, and rarely any requirement to exhibit high utilization rates...or deal with antagonists running on the same hardware.