Based on @patdavid’s explanation above, if I understand correctly:
Increasing the red slider in the red channel:
boosts the red channel (making the resultant image more red) based on the tonal information in the red channel itself.
Increasing the green slider in the red channel:
boosts the red channel (making the resultant image more red) based on the tonal information in the green channel.
I’ve not done the second type of operation myself, but I imagine it’s often a subtly different way of controlling colour. The more the green/other channel differs from the red/adjusting channel, the more different it will be.
It’s not blue that’s added to the red channel, but a portion of the blue channel’s tonal data/numbers. (The portion/amount added is controlled by the slider.) And so the red channel is increased as compared to the blue (and green) channel.