It isn't "off" per-se, but the context makes it seem "off'.
If you look at 1 John 1:5-10 as an address to those in darkness alternating with an address to those in the light it makes perfect sense.
A,B-A,B-A,B.
5,6-7,8,-9,10.
Light,-dark, light,-dark, light-darkness.
Both verses address those...