Well seems the London Lite today found a scoop on the Tube's DMI (destination boards with countdown times) and why there is a "mysterious difference between 'Tube time' and 'read time' ".
Now yes they arent based on how long before a train is going going to be at the platform but more how long it should take. What it fails to realise is that there is no way to really predict when the trains due. National rail time works the same way. After all it generally takes two minutes between Tube stop (very rule of thumb here). Now if your train is 3 stations away that leads to a 6mins time. Now it only takes an idiot holding the doors open at one of these station while hes mates jump on to delay the train. We've all seen it. Now how do you think that the boards on the platforms put info up? Crystal ball?
Common sense says it has to be a general X amount of time away but basic human interaction means that there is no way to to 100% right with this. The only time that LU can be sure when when its in the tunnel. So next time use your brain Lite.