Definitely blows my theory. Clouds as you mentioned can change the light temp. If something moved in relation to the light tent that could reflect a different light color and that could have an effect (especially if you were wearing a colored shirt OR... if for some reason you waited a certain period of time before taking the second shot (color temp of natural light changes throughout the day).
Cheers, Joe
You mention a shirt.. and this brings to mind an induced shadow by moving close enough to the gray target to 'fill up' the recommended area necessary for an in-camera custom WB.. at some angles no shadow, other angles a shadow. A bird flying over, wind moving a tree, there are a ton of explanations and they're all legit when you're talking small differences.. which goes to what DD was saying about 'accuracy', a word when working with color really needs to be quantified.
I think what works.. is for each person to understand the basics and then develop a style which fits them best. Location matters because of surrounding trees/buildings/mountains/people, weather matters, time of day matters (shifting angles for shadows), and of course a camera doing an in-camera WB could vary.. and certainly operator intervention in post can vary. So.. develop a style, pay attention to the details and variables, and do your best to be consistent.
I've found what works best for me.. is shooting the grey slab and doing an in-camera WB on location. I'd guess this is because my camera is sampling a much larger area than an eyedropper in post samples and perhaps uses a more sophisticated custom WB scheme.. and this could vary by camera. Then, my WB is correct for all subsequent shots including the color checker slabs.. so when building the color checker passport profile it's going to build the most accurate profile.
This might not be the best workflow for everyone, but it works for me. At least when shooting people towards the end of the day which is my habit. I might develop slightly different workflows in other circumstances.
I will say this.. I did some pictures of a boy wearing a bright blue shirt.. the sort of bright blue you never expect to perfectly match. The in-camera or post WB settings didn't quite nail it, it was still several shades off. Hand tweaking couldn't nail it.. and I've been hand tweaking WB for a long time like most of you. The CC Passport profile nailed it on the first try while keeping the skin tones exactly the same (to my eye) matching the color of the blue shirt. A significant difference.
This tells me where color is considered, there is a lot more than the greys/blacks/whites to color balance.. and is exactly the reason Xrite developed this device. It's contrary to our normal workflows and any conventional wisdom. Its almost like magic.. in that I can't duplicate the results in post without the profile no matter how many hours I sit there trying. I think this is because the profile is built taking the entire block of colors into account.. while manual adjustment we're using just a few.. or concentrating at once on just a few.
I encourage everyone to follow their directions to the letter.. in several different environments.. especially if there is a bright/odd color you're trying to nail.. and see how it works for you.
I would have scoffed at this a few weeks ago.. but when I wrote a review on the thing I tend to use the most scientific methodology I know.. and in this case I was surprised. All my other WB devices save or the big Kodak gray card I use in the studio have been tossed in the "not gonna use it anymore but I don't want to throw it away" box..