There's always the chance, in an acoustic performance, that the other instruments aren't actually loud enough for the vocalist when he's belting it out.
It's commonplace to use in-ear monitoring these days & once you've gone to the trouble of having your own personal, fits only you & no-one else, in-ear monitors - at anything from £200 to £1000 - then you probably get quite attached to how they sound on stage.
So, long & short of it - it's his personal mix, suited to how he wants to hear the overall performance, including himself & how he & the band sound from the mics, rather than just in the room.
If the seal isn't perfect - these things fit so snugly that you have to wet them in your mouth to lubricate them enough to actually go in your ears* - then maybe a slight adjustment is needed.
Additionally, there are a lot of nods, glances & hand signals, all very subtle, that an artist can use to ask the monitor engineer to adjust the mix. Some headsets also have a simple analog volume control in them.
*If you want to see this done, watch the build-up to any Formula 1 race. The drivers all use very similar systems & it's quite common to see the "lick'n'stick" of them putting the headsets in.