Maybe it was luck but the .243's breakup in the chest cavity put them down.I'm thinking of the energy being released inside the deer instead of beyond the deer after exit.
Don't sell the 243 short. It can be a very effective round. The largest bull elk I ever mounted for a client while I was a taxidermist for 20 years, was killed by a 16 year old with a .243. He was using Barnes handloads, and the elk went down with one shot throught the heart!