I have a Les Paul style guitar I am working on as a project. The setup is: two humbuckers, three-way toggle switch, two volume pots (one for each pickup) and one master tone knob. This is the way it has always been and I am happy with it.

Recently a friend of mine gave me a Gibson '57 Classic bridge pickup. I put this in the bridge position. The original neck pickup (Korean junk) has never worked, so I put the original bridge pickup in the neck position (just while I'm waiting for the new Gibson neck pickup). I installed it without flipping it over i.e. the six pole piece screws are still on the bridge side of the pickup. To me it sounds great like this.

The issue I'm having is this: when I have both pickups active, it has a very thin, out of phase type sound. I'm not good with electronics and circuits, but all I did was move the pickup. I didn't change any wiring at all.

Can anyone tell me why it sounds like this? Does it have anything to do with not "flipping" the pickup around? (having the pole piece screws to the neck side?) Like I said, it's not a bad sound, I'm just curious and people are asking how I get that tone...