Only 400 miles on a 105 cassette and as you claim well lubricated and not hard use would tell me something is very wrong. As far as using cogs too much I do big ring-small cog cross chaining frequently which would cause more wear and still get around 12k miles out of a 105 10sp cassette. I'm wondering if a used cassette was on there when you bought it. Could also be a freak factory defect from Shimano. Also do a google search for "105 11sp cassette defects" or "excessive wear" etc. and see if this is a common problem with other riders.