I'd hazard a guess that it's because the radars in TC use the LL QNH to determine when a/c pass 6,000 and therefore go on to FL's. If you are a bit below 6,000 (like at FL35) then the difference might be significant, and as we try to verify the Mode C it wouldn't be within 200ft. I usually use 'report your passing altitude', but then I don't work BB traffic. With the Solent Traffic (which is always climbing to a FL) I try to have an idea of the difference between Alt/FL based on the pressure of the day to know whether the Mode C is close enough or not.
Will see if anyone else corrects me!
Cheers,
N
"Keep smiling, it makes people wonder what you're up to...."