Selection tolerance in Channels.

Has he perhaps made a channel luminosity selection?  I always use Sven Stork's Interactive Luminosity Masks extension (I believe they are calling them plugins now) and I am not sure how the channel selection shortcuts work.  There does seem to be a difference between Ctrl clicking the RGB channel, and Alt Ctrl clicking the RGB channel.  It's easy to miss details in tutorials, and it is not unknown for there to be mistakes.

Could you both be using different Color Profiles? Perhaps the image has no profile and is defaulting to a different profile on your different systems.

I just ran a test. I duplicated a file and opened both in Photoshop. I assigned a different profile to each using the Assign Profile command within the Edit menu. I then held down Command/Control and clicked on the RGB channel within the Channels panel to load the tones as a selection. I jumped back and forth between the two images and the selections were different.

There is no tolerance if you're making a selection from a channel - ctrl/one-click in the channel icon. Can you post some screen shots? 

The Photoshop Training Channel put up an excellent video on using 'Calculations' yesterday.  This is one of the most powerful ways of using Channels to mask part of an image.  As is usually the way with these demostrations, Jesús Ramirez used an idealised image to demonstrate, and calculations will not work on busy images with multiple colours in the background, but it is a powerful and useful tool to have on hand.

Thanks for that Omar. 

I have actually got that far but my issue is I'm not getting much of a selection. I am wondering if I can reduce the tolerance so that I get more highlights selected? 

Thanks for your time 🙂

You mean to make a selection using the channels? If so, you can Cmd/Cntrl + Press on the Channel from the Channels Panel, the channel will be selected and the marching ants will appear 🙂