low compression is when the piston travels upward it squashes the fuel/air charge useualy when a engine has low com. its the piston rings worn and the charge is forced down passed the piston rings.This shows up as a fuel smell in the oil.Or the valves not sealing (valve seals)or(valve seats) I suppose the head gasket could be worn but this usualy shows up as loss of water from radiator or shows up on plug as a large lump of white crust some head gaskets leak very little water when worn and not blown so it could take months to see a low radiater. Some difference in compression is normal from one cyl to another in same engine.Hope this helps if i missed anything anyone reply as im not a mach.