18'' barrel velocity increase

I recently had the barrel on my xs7 .308 cut to 18''. I chronographed Rem factory loads @ 2738 fps in the 22'' barrel before I had it cut off. On Friday I shot a 5 shot group of these same shells, they averaged 3111 fps. I was amazed. I was hoping to not lose more than 200 fps but I gained 373 fps. Has anyone else had this happen when shortening a barrel?