How is points bonus calculated?

I may have asked this before, I forget, rings a bell...

Silver status, so 50% bonus. Just flown BA, a 100% bonus for silver is easily worked out. But for Virgin it was:

"Virgin Atlantic, LGWLAS Discounted Premium Economy 6528"

"Silver Membership 50% Miles Flown Bonus in Disc Premium Economy. 2611"

Ie: Not 50%.

It's worked out on the actual mileage (as in distance), not the base FC miles.