Why so different lumen efficiencies between mains and 22V halogen?
I should have researched more before asking.
On this page it is explained:
http://www.osram.com/osram_com/news-and-knowledge/halogen-lamps/professional-knowledge/high-voltage-and-low-voltage-lamps/index.jsp

The key factors are the physical properties determined by the laws of electricity. The coiled wire of a low voltage lamp is about five times as thick as that of high voltage lamp (mains voltage lamp) of the same intensity. However, the length of the low voltage coiled wire is just one fifth of the length of a comparable high voltage coil. The thicker, shorter coiled wire produced a considerably high thermal load of the low voltage coil. This leads to an increased luminous efficiency and a considerably longer service life.

Some more side effects of low voltage halogen are given here:
http://wiki.diyfaq.org.uk/index.php/Halogen_Lighting#Low_Voltage_or_Mains_Halogen

The light will be whiter and have better colour rendition (note that some may find high power LV lamps too clinical in some circumstances)
The filament in the LV lamps is also thicker (since it runs at a higher current) and hence more robust. This gives the bulbs longer life and also makes the bulbs less vulnerable to vibration (which will quickly cause a short life for any bulb, but particularly mains halogens).
The transformers and control gear used for LV lamps also typically includes a &quot;soft start&quot; capability that increases lamp life further by reducing the thermal shock the bulb experiences at switch on from cold.
The transformer will also typically prevent the high current surges that often occur when a bulb blows from causing the circuit fuse or MCB to also trip (a frequent problem with mains halogens)
LV lamps are also available in dichroic versions which are rarely found in mains voltage halogens.