I've never heard of UTG being "first position" or "position 1" before. I didn't design PT but I'll bet that the reason the seats are numbered this way is because different numbers of people at a table would mess up the statistics according to your system.
For example, according to your system, at a 5 handed table, the CO would be a 2 whereas at a 10 handed table, UTG+1 would be a 2. These positions play entirely differently and shouldn't be categorized the same.
According to the PT system, CO would be a 1 at a 5 handed table and UTG+1 would be a 6 at a 10 handed table. This discrepancy in positional number is a much more representative way of organizing position because number of people left to act is a significant factor in how you should play a hand.
Now there is some misrepresentation by comparing stats by position for a 5 handed table versus a 10 handed table to begin with but the PT system does a decent job of combining "like" positions when we want to combine all the numbers together.