70-72 dart fender ext.

So one thing I noticed is when I had to replace one on my 70 340 swinger the replacement I found was metal and the original one was fiberglass does anyone know anything about that.
I have both on my car, driver side is glass & potmetal on passenger side, I don't know why, the driver side fender isn't original. The 2 other 70s I've had were potmetal on both sides