Maybe because the frets on the guitar are further apart,and the guitar's neck is larger, so your fingers get to stretch more. When your ready to play the mandolin, your hand and fingers have had a good warm-up. It seems plausible--stretch four frets on a guitar, 1 finger per fret, then try it on the mandolin. I'm not saying the mandolin is easier to play though. My thinking is you have more strings on a scaled down instrument, it comes with a new set of issues, like accurately fretting notes on multiple strings, smaller fretboard, etc.