This is not a question about the mechanical effects. I'm just trying to figure out why 4e defines two effects that appear to be the same. If something is grabbed, isn't it in effect also restrained? If something is restrained wouldn't it also be grabbed by something? As a DM, I'm trying to figure out when/where I would use one effect over another.
You'd use restrained over grabbed in circumstances where the victim isn't just prevented from moving away, but also otherwise inconvenienced. (That's where the -2 and "grants combat advantage" come in.) Consider "held firmly by the arm" and "wrapped up in animated vines."
If I've got your wrist in one of my hands, you're grabbed. If I've got a rope around your belly, again, grabbed.
If I've got both your wrists behind your back, you're restrained. If my rope is pinning your arms, you're probably restrained.
Grabbed - in simple terms means to "hold" or "stop" something. Restrained is more of "restriction" type wherein the object is not able to execute a particular action