This week's book giveaway is in the Agile and other Processes forum. We're giving away four copies of The Mikado Method and have Ola Ellnestam and Daniel Brolund on-line! See this thread for details.
Without any info on which servlet version the deployment descriptor is targeted, the container can only guess. It will usually guess that you are specifying the latest version that the container supports, but that may not be correct, and probably won't be if you upgrade the container or move the app to a different container.