If it shoots that good I wouldn't care. The paper doesn't lie.
And that is what really matters. I can understand solving this puzzle because it gives you confidence in your next shot. But in the end if you are getting the accuracy you want and expect at the ranges you require then you can relegate the SD spreads to a lesser priority in your load development.
I went down this same rabbit hole back in my old competition days. I would take a perfectly fine shooting load and totally screw it up by constantly changing darned near everything I could to get the SD into low single digits. I finally learned that single digit SD is not necessarily a requirement to accuracy shooting - but trigger time is. It took a lot of lead and powder for me to learn it though. Check out this wikipedia link so you are sure of what you are trying to solve, http://en.wikipedia.org/wiki/Standard_deviation
So then, the list of things to try is long, and in your case some are limiting (such as COAL), but here are a few things you can try to tighten that SD.
1. Cartridge OAL
3. De-burring the primer flash hole
4. Case neck brass thickness uniforming
5. Uniform brass neck hardness (annealing)
6. Indexing brass, but may be somewhat impractical in a semi-auto
7. Weighing brass
8. Only shooting development loads on same temp/density altitude days
9. Changing lots of powders
10. Changing powder
11. Add more to the list that I can't think of right off the top of my head
I didn't go back to my notes, and these were not in any particular order, but you can start to see the level of absurdity I was going towards that never materialized in a better score or more silhouettes hitting the dirt. I reasoned that the proper way of going about this is to completely eliminate every variable YOU can eliminate, and then only modify those things that you cannot. I even went as far as developing loads through one single brass case. And in the end, all I did was get tired of it. I got my SD into single digits, but the accuracy wasn't really any (or much) better than it already was. I chased my tail right into the hole of diminishing returns.