A puzzle

I developed three rolls of film this evening. All were disappointing. It seems like all three were underexposed by about 1 to 2 stops. That's odd because I don't think I've changed my metering. Two rolls were Arista 200, one was Neopan 100 so it's probably not film related. The only other thing I did differently this time was use Xtol stock rather than 1+1. Whether they are too thin to print I don't know yet, but there's obviously a lot of shadow detail that will be lost. I hope it's not my handy calculator. Two steps forward, one step back... or is it the other way round.