Can't be sure, but possibly down to a known glitch with the icon library (which gradually evolved from 1.2, restricted colours on early Amigas to 256 or more colours on Workbench 3 and later machines).
Workbench 2.1 also had some some support for 24 bit card displays and DCTV and similar graphics upgrades, so you really only encounter it on later versions of AmigaDOS, 3 and up.
This thread might offer a workaround, but from what I remember, there are various modded versions of icon,library around to address this issue.
http://www.amiga.org/forums/showthread.php?t=70439
