This may be over simplification......Think about making veggie soup in a pot at home, all of the heavy stuff (potatoes and carrots)goes to the bottom, the cooked onions(light stuff) float to the top. When the puddle is molten, the heavy stuff(metal) goes to the bottom, the slag(lighter stuff) floats out on top. But...this doesn't explain out of position welds....

Here we go.

You hit the nail on the head when you mentioned:
"the tendency of slags always to rise to the surface of welded joints"

Slags for SMAW and FCAW are non-metallic.  When the weld metal freezes the slag is pushed (ejected) from the liquid solution. (just as the gasses are ejected from the liquid solution)

The reason the slag goes to the surface of an overhead weld (the bottom) is that it is being pushed out of the metal. Gravity is not a factor. 

The liquid weld metal is in a free solution with the slag, the non-metallic slag does part of it's job combining with impurities and oxygen from the liquid solution and then is expelled by the force of the freezing metal (nucliation and dendrite growth).

So while the slag appears to be floating atop the weld puddle even in the overhead position, it is in reality being pushed away from the weld and base metal where it quickly freezes and does other parts of its job (holding the bead in place to stop gravity from pulling drips of hot metal and controlling the coolng rate.)