Rightily-dightily there Ned.
The Defender weapons could shift their total pluses about from to hit and damage, and AC. In fact there was a +6 defender in 1st Edition D&D, which seems a bit sick to me.
So...Drizzt's scimitar defender is indeed a +5 weapon, but it is frozen with 2 pluses in AC, and 3 in to hit and damage.