Alzner began the season as a 1LD, a line shuffle in the middle of the season took him away from Carlson to try to create two pairs that played around the same, but by the spring/playoffs he was a 7D that Washington did better the less he played.
Of course when you average that he looks like a first pair D, and not all players should be judged by how they end the season (people did that with Eric Staal in New York and that's why Minnesota got him so cheap), but Caps fans watched Alzner slowly deteriorate and play games he shouldn't have due to injury, and try to be an iron man, only to finish the year damaged goods that they no longer wanted. And Montreal signed those damaged goods like the Maple Leafs signed Komisarek in 2009. They paid for a reputation and not an actual product.