It makes zero sense. Who the hell thought up of 42.195 km or 26.21876 miles? It's not the exact distance that Pheidippides ran, nor is it easily expressed in Imperial or Metric units. The half marathon is even worse:
"Hey, what did you do this weekend?"
"Well, I raced a 21.0975K and won."
"How far is that?"
"Oh, about 13.1094 miles, but not exactly..."
Make it a nice round number like all of the other race distances. Either exactly 40 km or exactly 25 miles. Both are better approximations of the actual distance from Marathon to Athens, and you'd see a lot more sub 2s.