Why is mains power sometimes 110V and other times 120V?
In the US, the electric utilities are supposed to deliver power to residential customers at anywhere between 110 and 125 VAC RMS. The value 117 (or 117.5 or 118) is often seen on products, because that is the middle of the specified range.

If you're developing a product for general sale, it would be prudent to add a testing margin that's at least 5% or even 10% beyond the nominal range &mdash; perhaps 100 to 140 VAC RMS.