We were good against Brighton, but not great. It showed what we can do when some things go our way.
Southampton play a style, particularly in the first half, that we struggle with because we're more or less incapable of playing through a good high press. When Brighton did similar in the second half we also struggled, but that 3-1 goal dropped their momentum.
We also had phases and moments where we didn't defend well against Southampton, not as much against Brighton.
Southampton rested players in the previous game.
Inconsistency, yes to an extent. But imo it's mostly other factors.