Because in 2006 I had 2 claims on my car insurance which has pushed my Bike insurance up without the added benefit of being able to use my 6 years NCB.
Hence im under the illusion that staying below 600 will keep the premium down a bit that's the main reason.
Check with Bennets - see what they say, cos the difference may not be worth blinking at. Esp when you may end up paying more dosh for the bike as a 400 than a 600 due to there being less 400's around.