If I had to make an educated guess... Repair cost is worked out by how well players do in battle. By rank 5 all the 'bad' players have been weeded out of the British tech tree. Those that are left by now are committed and skilled in equal parts. The Russian tech tree on the other hand is far more forgiving and easy to play, resulting in a higher ratio of those who've just herpaderped their way through the entire tree and haven't really learnt anything. These players drag the average down resulting in lower repair costs.
Sparkbox replied to Blitzkrieguk's topic in Ground Forces DiscussionAn interesting point of the FV4005's shed turret is that it rarely if ever trigger's APHE. There's been more than one unlucky sod that's suffered some hellish return fire when their all-powerful APHE nuke round failed them.