I manipulated NBT data a bit to spawn anvils with mob spawners. Problem is they always miss one row on X and Z axis.

As english isn't my native language I made a video showing the issue. It's to large to upload as attachment, so here's a link: https://vimeo.com/55709238

Also I try to explain: Let's say the spawner is at x 6 and z 6. spawn range is 5. The expected output is to add 5 to all sides, so spawn from 0 to 11 on x and z (5 + 1 (the mob spawner) + 5). The given result is 5 + 1 + 4, so from 0 to 10.

BTW: This happens with all entities, anvils are just good to show it. This note is important as it says that this happens with vanilla gameplay, too.