I'm not sure HOW luxfer got that number for a 68/4500. They were correct on the metric information page for everything else (68ci = 1.114 liters)
311 Liters works out to 62/4500. The only way I can see this being true is if the 68 cubic inches is the water displacement of the tank, not the water capacity, which would explain the "missing" 6 cubic inches being the bottle itself due to the wall thickness.
Luxfer has (rounded) 11.0 CF of air listed with the 311L for that tank. 11.0 CF = 311.487 L
Here's an excerpt I found on Metric vs Imperial measurements of scuba tanks:
Metric--with its ever logical approach--measures a tank's capacity in liters of water that fit inside. And since a liter of water also so happens to weigh one kilogram, a tank's capacity may be given in kilograms.
Imperial rates tanks by how many cubic feet of compressed air it can hold at sealevel. That makes the imperial rating much more relative as capacity in cubic feet depends on the tank's rated pressure as well as temperature.
This makes more sense with the 68cubic inches being the displacement, while the 1.1 liter is actual capacity.. the decimal difference would be the 6 inches.