We do know what the difference is. The habitat conditions have improved substantially, allowing the animals to successfully reproduce in that area. We are less efficient in controlling sea lamprey there because the chemical we normally release is very efficient in small streams, but we're dealing with a body of water that's 300 metres wide, several metres deep, and moving very fast in some locations. You would need a humongous amount to be effective in killing the larvae, and most of it wouldn't make it all the way to the bottom.
On April 30th, 2012. See this statement in context.