Applying current to glow plugs for a fixed amount of time (eg 10 seconds) will presumably not take them to a temp (eg to 80 degrees) - it will presumably raise their temp by a number of degrees (eg by 60 degrees), so if ambient temp is 20, it will take them to 80, but only 55 if ambient is -5. Presumably if the weather is very cold, the battery may perform less efficiently and therefore the rise in temp may be less - eg a 50 degrees rise so at -5 it will only take it to 45. (figures all examples)
I know my figures are only guesses and show a big difference in temps, from 80 down to 45 - but the principle is probably correct. The designers though will (surely!) have taken that into account and have designed the system so that the glow plugs are live for the "correct" amount of time. I don't think I've ever had to double of tripple pre-heat the glow plugs on my L Series - and I have started it when it has been below -5. I have started the car, "chiseled" the ice off the car, driven to my fishing spots, tackled up and even after that length of time the line is still freezing to the rings on the fishing rod - but the L Series started on the first turn (I'm sure the TD4 should be the same).
If the glow plugs are old, corroded and/or sooted up presumably this would reduce their efficiency - which may take them outside the operational expectancy the designers worked with - hence they might need 2 or more pre-heats to raise them by the same temperature. I would have thought this is particularly unwanted as it is the very cold morning when you'll need to pre-heat multiple times, which will drain quite a bit of power from the battery, and its also the time when the starter motor will needs as much oomph as possible!
