Changing Colors Using Curves and RGB Channels

Hi all --

I use the red, green, and blue channels in curves to change an object's color, and it usually works, but sometimes it won't and I can't figure out why. For example, I want to change a color swatch to R 154, G 61, B 55. I added a color sampler to the image so I can see the RGB values in the Info tab.  I change the color in the curves output box for each channel. After I do that, usually the Info tab shows the values I input into the outboxes. For example I'd see R 154, G 61 B 55. But sometimes the numbers are a little off (for example, R 156, G 63, B 57), and sometimes they're not even close.

Anybody have any idea why it does this? More importantly, does anybody know how to make Photoshop not do that? I've attached a screengrab showing the above example. I'm using Windows 11 and Photoshop 25.11.0. I've had this same problem for a long while with many different versions of Photoshop. 

Scott