My guess is you are going from RGB to CMYK. Two different color spaces. So, if you call up a Spot 300, I get 0_R, 99_G, 186_B. Convert it to CMYK, I get 100%C, 54%M, 0%Y, 0%K. Now, convert it back to RGB = 39R, 94G, 159B, then back to Spot = 2945. This is common drift from one space to another and then back again. Totally expected. Sort of forces you to remain focused on what you are ultimately trying to achieve and not go back and forth willy nilly. Why are you doing this?
The question is not why. I am not going back and forth I am going from RGB to CMYK not back again. If this should not be done it would not even be an option but in this case it is. The big question is once I am in CMYK why can I not use the correct Pantone color. I select color A and it apply it while in CMYK mode and go back and check the color and it is now color B not the color it was just changed too. I never go back to RGB mode.
Like the previous poster, I do not understand why you're doing this. Not having that insight, it's impossible to comment further, especially given that post #1 already tells you what is happening.
Maybe I'm misconstruing your post. Sorry.