I Went To Fill Up, And I Noticed This

Is there a reason I would see this?

Um which part exactly? The broken lanyard from the OEM cap, the locking gas cap, or the dirt and crud that gets collected from a dirty car and rain running into the seams and collecting? I always detail within the gas door when I wash

Yeah, you need to be more specific.
Did you fill up early morning on a hot day? The dirt looks like it has stuck to spilled gasoline-perhaps that locking gas cap isn't sealing properly. Also, you can pop the plastic tab out and get rid of the lanyard as it isn't doing you any good just hanging there.

I always wash under there

looks new

It's the blackened crud that I was addressing. I've seen it before right below the filler nosal when I've accidentally spilled a little. However, I've never seen it on the door before.

Thanks for the tip on removing the lanyard too.

It should take years of doing a crappy job filling you gas tank for it to get that dirty. Unless you live on a dirt road. I would look into changing the gas cap and see if it still gets that dirty.

When's the last time you hand-washed the car...that looks like it's been there awhile...???

Easy to clean and will look new when done.