1) Yes there is a difference, the higher the voltage the brighter the LED gets. However, the higher above the actual run voltage the shorter the LED will last because it will burn out. You can buy LEDs that are made for the higher voltages and they are amazingly bright, however they tend to cost a lot and tend to need their own heat sinks because they got hot enough to melt lexan without them.

2) .2 v is just a number that i use as a rule of thumb for plus or minus errors in making cheap electronics. Its a number in this case that won't really make a difference if it is between the plus or minus .2 v.