This is the benefit of the 3 in midfield. Allows Henry and Brannagan the freedom to get forward whilst the other sits slightly deeper helping out the DM. In the 4231 he can't do this because you expose the centre back behind you.
Check back to the extended highlights of the Plymouth game (you don't see it on the 2 minute version). Sublime bit of skill from Brannagan inside his own half to pretty much take three Plymouth players out of the game, before running deep into their half to set up Mackie for the first goal.