Until Lima was extended, 27R was just to the left of the only full-length taxiway. If, for whatever reason, the dark runway didn't stick out, but both Lima and Mike did, then Mike, being the dirty darker straight line left of the shiny new Lima, would look a lot that what you used to expect: 27 R, just left of the lighter color Lima.

Makes a little more sense after going through ATL for the first time in a long time...