First of all, in the absence of the introduction of the ground wire of the two-wire plug of the switching power supply system will have such a situation.
In order to suppress common mode interference, the switching power supply in the design, the "safety capacitor" will be between the "LG / NG" (safety capacitor is used for such occasions, even if the capacitor fails, will not lead to electric shock, so the secondary side of the switching power supply "ground" is equivalent to the midpoint of the AC voltage, so you measured 110V voltage. But this does not mean there will be security risks, after all, the capacitance capacity of only 1000pf.
If you are still worried about its leakage, then the correct way is to measure its DC leakage resistance, the use of insulation tester is the most effective way