I think that’s down to personnel available. At the start of the season we set up as a 433 (Bielik wasn’t match fit, Holmes was injured and Shinnie not favoured) - Evans picked you an injury in the first couple of games meaning Huddlestone was our only available DM... 433 made sense.
We shifted to 4231 when it was apparent we weren’t going to get a performance out of Dowell and Holmes was just returning from injury - Bielik has gained some match fitness but we didn’t have any fit AM, so we used Paterson as a sort of second striker instead... 4231 made sense.
Recently, Waghorn was struggling for form, Jozefzoon was as well (may have had a small injury too), leaving us with Lawrence and Bennett who wasn’t playing too well either... this meant 442 flat/diamond suites the players available most.
Chances are we’ll be using one of our DMs as a CB after the international break and we’ll revert back to 433 due to who is fit to play. With Rooney joining and Sibley/Knight improving, I’ll be surprised to see us line up with a 4231 in the second half of the season.