I've got 2 LG4163B's & get slightly different results with these, they're minimal however(& probably due to temp/ &/ or your computer doing stuff in the background)
In the UK the power from the national grid varies at different times of the day, this is deliberate in order to meet demand. (Therefore voltage will rise & fall for example from 240v to 220v)
This has almost no affect visually unless making some sort of performance test.
In answer to why you get such a variant between your drives. The LG is set to read at 4-10 speed. Whereas the NEC is set for 6-12 speed.
The reason why there's a difference has to do with error correction. As drives get faster at writing so the possibillity of error's increase, but because when reading these same drives have erros correction this should be negligible.
LG's read speed is playing safe unlike NEC( & PLEXTOR) Try a pressed disc & you should find both drives read as high as 6-16 for both drives.
PS: I'm using the latest firmware, but future firmware may improve read speed. Only time will tell.