I asked Trek why they decided to offer the multiple medium sizes, and if that policy might be extended to other models. Here’s what Michael Browne, Trek’s Mountain Bike Brand Manager, had to say:
We saw a need and filled it. The 17.5and 19.5 are our most popular sizes, but we continued to hear from dealers and consumers that either one was too small or too large for many people. So, we decided to make one that was just right.
It’s been received incredibly well, and riders can expect this new size to expand throughout most parts of our bike lineup as we move forward.
Bonus picture of what my actual test bike looks like in full battle regalia, including a bit of Western Pennsylvania trail splatter: