cyclocross sizing

I've heard that for a cyclocross bike, one should pick a frame size smaller than they're measurement. is this true ? can anyone elaborate on this for me ?
according to wrenchscience.com fitting calc. I would normally use 55cm frame. I'm 6' tall weighing about 165 lbs. does this sound right ? I'm looking for a Trek xo1 or maybe cannondale xr800.

 

What's your inseam, measured with the style mentioned in bike fitting guides?

 

measurements ( in inches ) are as follows:

inseam: 35.25
notch: 59.5
height: 72
arm length: 25.75
shoulder width: 17.5
flexibility: 4
weight: 165 lbs
shoe size: 10 ( US Mens )

thanks, in advance, for helping a bumbling novice

 

I was told this is so because it is easier to mount and dismount during cross racing on a smaller frame.

I had a Trek XO1 but I only used it for road riding / commuting and I picked my normal size because it was the most comfortable. I never raced any cross so I can't elaborate on racing. I would talk to people with cross experience if you are looking into racing. If you just want a bike for normal riding, get your normal frame size.

 

oops, I screwed up the wrenchscience.com measurements. according to it I should have a 62cm frame ( center to top ).