Reframing the idea behind “Alpha” as occlusion should make the idea clearer, where “Transmission” is a degree of filtration.
The 0.0-1.0 range in alpha represents the degree of occlusion from 0% to 100% in a properly encoded image, while the RGB represents the quantity of emission. Something can be completely unoccluding (0.0 alpha, 0%) and emitting (nonzero RGB units), as well as completely occluding (1.0 alpha, 100%) and not emitting (0.0 RGB units).
Transmission represents the ratios of light permitted to pass through the object. 0.0 would indicate that 0% of the given channel is permitted to pass through, while 1.0 would indicate 100%. In this way, Transmission behaves like a light “filter”.
In this way, the RGB values are pulling double duty, both as the emission for the alpha component and as the filtration mechanism for Transmission. It is worth understanding that in different contexts the values present in the three channels can mean different things, and it's not always a "colour" per se.