I'm not surprised. Your guitar has almost twenty years additional aging in the wood. It is also a rather uncommon model which probably means it was built with better woods, more care and by more experienced luthiers than its strictly production-grade 14-fret cousins. And though the '70s in general were "slump" years for Martin, '71 was early enough that the downturn in quality was still a few years away.

Also, when playing one instrument most of the time one's playing style tends to undergo subtle little changes to accomodate for an instrument's weak points and accentuate its strong points. If you play a bass-heavy guitar, you have probably developed a bass-heavy playing style. When you play a guitar with less bass response you really notice it because your playing style doesn't work as well on it. Meanwhile, the owner of that guitar that doesn't sound so hot when you play it has developed a style that accentuates its strong points so that it sounds like dynamite when he plays it.