Let’s take a look at a chart of the winning times in the Boston Marathon for the Open and Wheelchair divisions:
Note how as fields get larger and more experienced, much of the variability from year to year goes away. The trends match the trends for the marathon community as a whole, represented below by the lines representing the marathon world record:
When we take a closer look at the modern era (from 1966 on), we can see that the year-to-year variability tracks across divisions, moreso as race field size and quality stabilize:
Clearly common external factors (mostly weather, I assume) have a visible effect.