You view posts "sparsely," that is, you tend to view posts with non-contiguous ID numbers. Rather than storing each number, I'm storing a range that says "This user has read from post #20 to post #30." Each time you read a post near that range, I can increase the range by one. But if you view a post that's more than 2 away from that range, I have to store a new range entirely.

The more posts you read, the less "sparse" your viewing history so be. That you have more ranges than anyone else just means you don't make it a point to read everything, just things you're interested in. That makes you a discerning reader. :)