I don't know if this analogy is valid but...
Dips on a dip station < dips on rings.
I can do about 20-30 dips on a dip station (where the machine doesn't move) but I can only muster about 4 dips on rings, where my stabilizing muscles are all out of wack and working hard just to keep me upright. Technically, I'm still lifting the same weight (my body) and doing a similar movement (the dip), but the muscles involved seem to be heavily under-developed when it comes to rings than when it comes to the dip station/parallel bars/etc.
That's kind of how machines and free weights seem to me.