Voltage drop through the wires and relay contacts can account for the drop at the glow plugs. Your best point of refference is comparison numbers with another simular car. The manufactured does save some money by not really making some wires large enough to deal with the heavy currents quite often.

Even with a fully charged battery eleven volts at the glowlugs is about the highest reported I ever remember hearing. Yes the glow plugs are rated at 12 volts. They just never see it unless perhaps after the engine is running and they are still on. You may only have 8 or nine volts at the glow plugs. That is a little too low.

A good battery should be resting after several hours at 12.5 volts or slightly higher. An older battery will have more internal resistance that lowers the output voltage even further at the battery under heavy load. Mind at the glow plugs.

As a first test I would start the car and read the voltage at the battery at say 1/3 rpm. The charging voltage from the alternator may be low. That could be reflected in the battery being undercharged. If it is 13.8 volts or higher and the car is in proper use the next stop would be to test the battery at a quality free checking battery place. It could be getting tired instead..