Well, RES is physical memory, it's just some places where it gets reported don't account for physical memory that is being shared between multiple processes.
If I have a process that says it's using 1GB of RES, then you can guarantee there is at least 1GB of physical memory in use. It just might also be in use by other things also reporting their own RES usage.
smem's measures - USS, unqiue set size (unshared pages) and PSS, proportional set size (shared pages divided by their share count, ie. how many tasks share them), are far more accurate and easier to reason about than RSS and VSS.