Quote - ... a big aperture setting takes an average of the color. A smaller aperture gives a more precise reading.
... a second article which may be of interest to you. It explains an inherent problem between Photoshop’s color management system and the OS’s. Just thought I’d post them for your perusal. On the second site you may want to get right to the point so you could start reading after the 8th paragraph where it says: “Why is this so difficult?”

mac tricks and tips

smashing magazine

Thanks for the links!
You're right, the size of the aperture on the digital colour meter does effect the reading.  I've tried it small and large - both give me the wrong settings for the colour I want.

I'll take a look at the links you posted and see if I can use the information.