I have seen people wear out selective cogs on their bikes before like you describe. One guy must have only weighed like 130 lbs but he insisted on only riding in the 11th tooth cog and he went through that cog before he went through the chain. Honestly I just think it is your riding style and you are probably just grinding down those 2 cogs because of overuse. Remember that the estimated mileage of a cassette is based around the idea that the wear has been spread over a number of the cogs.