Mal: I'm 99.99% sure it wasn't like that when I first reloaded it. I was ultra meticulous doing my first rounds and I'm positive I wouldn't have missed that.
That case with the big hole definitely had an extra large amount of scorching on the outside, which tells me it definitely fired differently. What might have happened - lower pressure round the mouth to not expand properly? I would have thought the larger flash hole (caused during this firing or not) would cause higher pressure.
But, as usual, thanks for the advice fellas!