Like Pantone books, color checkers don't last long before the colors fade, maybe a year or less. I remember being surprised at how much money we were spending on them.
I don't know anything about it, but I find this interesting, perhaps slightly surprising. It doesn't suprise me that it would fade, but, that being a presumably known problem, do the color checkers not have a good cover/case for storage? (something foil backed or similar to really block out the light)? Or do they even fade quickly just from being taken out briefly for use?
The colors may also fade/drift upon exposure to air and humidity, not just light.