It makes perfect sense. Starters having more pitches to fall back on. Relievers usually only have one or two good pitches, so the pitcher has less options to make adjustments.

This lines up well with what Jeff Zimmerman and I found regarding pitcher aging and how it differs depending on a pitchers role.

Let’s take the example of strike outs. Jeff and I found that while starting pitchers were able to mitigate against their decline in velocity–and therefore experienced a less drastic decline ...