And there is a rules option, by Jason Rahman published as Daily Content that suggests that atrs defending in an assault hex against armor can be assumed to be in crossfire and should received a considerable on the spot increase to their anti-tank value in that situation.
http://www.avalanchepress.com/Rifles.php Would have liked for this to have ended up in the 4th edition rule options.
Can't ask M Perryman what his reasoning was, he designed PG Citadel many years ago and his reasons are lost (if the difference wasn't added during development by Doug McNair, who is out of the loop now).