It seems as those teeth curl around and flatten against the retainer as it's driven in. I would imagine it keeps the bearing from walking back and forth. Pound it in and start driving.
There's no rhyme or reason for all the teethies on the seal to not match the retainer slots.
The old seal was factory and was identical to my replacement. The only thing I could mention is that the mark on the housing and the mark on the retainer lined up fine when tapping the retainer back in.
I tried every slot on the retainer, turning the seal one toof at a time and I always ended up with 1 or 2 of the teefs that were not going to squash down.
New seal seated just fine.
Just FYI guys, cause many Chev/GMC 4x4 including Tahoe etc are apparently the same deal on the left.