I've noticed the strangeness on some guitars with light strings and I'm pretty sure it's down to the height of the frets. I've some guitars that have very low actions and buzz free, but when my clumsy playing has me fret a note the bend of the string to it's maximum when my finger touches the fingerboard is quite low. On a guitar with higher frets, pressing a string down till it hits the woodwork sharpens it more and it becomes out of tune. with thicker strings my stumpy fingers stop pressing when the string hits the fret. with lighter strings it goes down further. As a consequence, I use heavier gauge strings in higher fretted guitars and the lighter ones only on low fretted ones. This could be what is happening here maybe?
