Overall DTR Score is not calculated by the sum of your individual characters. It's calculated using the sum of your characters' stats. Think of your overall score being a fourth character that uses all of the stats in the DTR Score calculation.
We do this because simply providing the sum of the three characters would yield an advantage to players who play evenly across their characters. DTR Score becomes more difficult as earn more, so reaching 150,000 total site score by adding three characters with 50,000 each would be easier to acquire over trying to get 150,000 on a single character. I hope that makes sense. By using a abstract fourth character, we can more accurately calculate DTR Score regardless of how many characters a player may use.
For more information related to Site Score, give the Site Score FAQ page
a visit. If you still have questions regarding Site Score, drop a reply here or in that thread and I'll get back to you.