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."