ASAIK, 117-120 v is "standard". There are several devices (Monster AVS-2000 is one), that will put out a constant voltage.
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How are you measuring? To neutral or ground? The difference might be due to something running to the neutral. Should be referenced to ground.
American residential has 2 phases of "115" volt, cumulative to 230. Possibly an imbalance between them that can happen for several reasons outside of the house. Some outlets would be on one of either phases. Go for the higher voltage if you can.
A step-down transformer can moderate the difference between the two phases if hooked up to both but it will probably hum due to compromised power.