Actually, it does make sense if you remember how APFS works. The Container has a certain capacity. Each Volume in the Container can use as much of that space until the Container is full. So if you have two Volumes, both will show a capacity of whatever the Container has available. As one or more Volumes begin to use space, ALL of them will show the decrease. So think of the capacity of a Volume as the remaining capacity of the Container and it makes sense.
Both had the same capacity (each the size of the SSD capacity, which made no sense!), but one was empty.