It looks like the dealer forgot to put the plastic pieces that come with the spoiler under the base when it was mounted, look at the picture Last1 posted and check out the difference.
I put mine on my 2010 and it makes the spoiler sit level on the trunk and fills the gap that yours has.

 

Maybe the dealer didn't bother drilling the extra 4 holes for the forward end of each pedestal, and maybe even didn't install the double-sided tape at the base of each pedestal. I've personally installed two OEM pedestal spoilers with no issues like this. They even come with a template to get the holes drilled in the exact location needed.

Here's a thread showing the one I installed on my 2012 Boss.
https://themustangsource.com/f813/ne...poiler-503865/

And another thread that should help.
https://themustangsource.com/f813/new-wing-494586/

 

The holes are there, and I believe the tape is too, but I haven't verified that 100%. The rubber bases are certainly missing, however. I'll have to call them Monday.

 

There are no rubber bases, just precut double-sided tape, which is maybe 1/32 to 1/16 thick.

 

Mine on my 2010 looked just like Last1's did, I don't have it now but it was the bottom piece the double sided tape was on. The bottom piece of the spoiler on mine had raised sections that actually fit into those pieces which is why his sits a little up of the trunk lid.

 

There's an AM video on YouTube regarding pre painted pedestal spoiler for 10-14. It's definitely a replica as they spray the entire thing body color.

The OEM is two part and fused with black plastic. Id check with folks who ordered it from AM and confirm it's quality and fitment before you plunk the coin.

 

I might try and buy some thin rubber and cut it to fit the bases, see how it looks. It'll probably take forever but it should look better, I think. I can touch up the black parts and make that look nicer as well at the same time.