As far as the barrel length goes, the 20" was increased to 22" for the Short Action Mags. They need more barrel length to burn up all the powder that's in the case.

Don't know why they went from 18 1/2" to 20". The shorter barrel is what they put on their carbine length 7400s so seems like there should be some commonality there.

I've read that the Model Seven balances better with the 20" barrel but I still like the 18 1/2". Mine is chambered in 7mm 08 and it's a joy to hunt and shoot with.