It's not undersized piping, but long runs or radiation with more pumping head. It's legitimate to have radiation flow & delta-T requirements that differ from those of the boiler. The boiler manufacturers remain agnostic of those other system requirements, and can't assume.

Most single-zone systems COULD be done with a single pump, but not always the one that came pre-installed with the boiler (even though many or even most would still be fine.) But you DO need to do the math on the system to know for sure, a skill that has gone over the heads of too many "heetin' an' plummin' " boiler installers, contractors of the ilk who would recommend a 150KBTU/hr boiler for a house with a sub-25KBTU/hr design day heat load.

Given the facts about the average design expertise of the installers, it's no surprise that some manufacturers automatically void the warranty if it isn't plumbed P/S. (But apparently Burnham allows those who can do the math to show off their skills!)