At a certain point the head angle needs to change to match the wheelbase to bring about equal handling to other model sizes in the lineup. Apparently SC decided that the XL was the place it needed to happen. I don't think they would have gone to the trouble of changing just the XL unless they found a fairly negative issue in handling, climbing, etc. if left alone. They build and test prototypes, and many "spy" photos have been seen on mtbr and in magazines of different SC bikes. If I recall the XL was the last frame to make full production, and it may very well have been because of the changed head angle. I'll be they did it for a good reason. You might think that a 1/2" wouldn't be much of a factor, but something came up for them to change it. Maybe it was a bad climbing or cornering trait, or something else. I'd want to hear from other XL Nomad riders before I dismissed the head angle change as being a negative trait on this size bike.
This issue reminds me of a major geometry change issue that occured in the dirt motorcycle world around '80. To get stability, dirt motors just slackened the head angle. This started negatively affecting handling to a noticeable degree in many areas. Then they got the idea to lenthen the wheelbase and sharply steepen the head angle. Magically they got the best of both worlds with this setup, and it's still with us today.