And the age old answer of... grounds.
Depending on the quality of the ground you will get different voltages. A bad ground that is a volt higher than another ground will be a volt off. Is it a volt higher or lower, that depends on what reference point you are using. If the voltage regulator has a ground that is at the higher potential than the ground on the Sniper, the sniper will read a higher voltage. The Sniper really is getting the voltage it is reading.
My first Holley EFI I fought drivability issues. But only at night. One night I was watching O2 voltages, shut the engine off but had the computer on. Was calling it a night and turned off the headlights. Saw the O2 voltage change. Headlights back on, off, on, off. Every time I turned the headlight switch, the O2 reading changed. This was with the engine off. This was the days where the O2 ground was on the exhaust, but the ECM ground was on the body. Move the ECM ground to the engine, common ground, fixed. Good thing is the Sniper is better and the O2 ground is grounded through the harness to the computer so there is not wild ground IN THE EFI. But that doesn't mean there isn't a ground leak between engine/body/battery/etc.
Now there is one more thing to check. The voltage regulator has a reference wire that goes straight to the battery. This is what the voltage regulator is looking at. If there is a voltage drop in the harness, the voltage in the harness will be higher. The regulator trying to keep 14.2 at the battery can make 15 or 16 volts at the back of the alternator to allow for the voltage drop through the harness to the battery.
