The white channel is all of the color channels (RGB) combined so it’s a vector (float3). The single channels are only a single float value for each “part” of the RGB color if that makes sense. Your AO map is only using grayscale values so it can be displayed using a single float value. And it seems like in this case the blend node was made to only work with grayscale values.
Thank you for your clarifications ,i understand the channels and all you said but when i opened the texture editor of the AO map it seams like each pin red ,green and blue holds the same ao texture ,and there is this confusion about channels like in quixel and substance painter sometimes they refer to albedo ,metalness,roughness as channels at the same time the R ,G and B they hold also called channels , so they call a texture map a channel and the same time the components of it R,G and B Channels ,so i’m a little confused about that.