It makes sense really. There were 2-4 main Smallpox strains right up into the early 70's. The last natural cases reported were of the least virulent one, the one with the highest chances of survival.
So why not 2-3 main strains of Yersinis pestis?
Alternatively, given the general state of nutrition and hygene a few centuries ago a disease which seems weak to us now could have had more leeway to be a problem.