Stats are random, but are influenced by character level. The higher level you are, the better chance you have of getting something in the upper range of possibilities for that item. I did the Dark Forest north to south, so may have been at a higher level when I got the silver armour, making it easier to get a 410 armour class.
If you quick save after talking to Swen, you can quick load repeatedly to identify the Larian armour he put in your inventory. I don't think there is a huge difference between 360 and 410; depending on the other stats for the 360 armour, how long it takes the game to load on your computer and how often the maximum armour class comes up, it may or may not be worth it to keep re-loading.
