Coors field adds about 1.75 runs for TT. You can break it down to day game vs. night games, and even home games by month over previous years (factor in humidor) to get more accurate numbers. But the gist of it is you aren't looking at more than 2.00 runs for TT.
The reason it is low is because Clemens gets poor run support. This is the same problem as the Big Unit syndrome where offense always shuts down thinking they only need to score a few runs to win (same drill with Pedro & Schilling in their prime). It's all subconcious but if you look at the average runs support for these aces receive vs. the rest of the team for each year, you'll see a big difference in run support.
Houston has averaged only 2.87 in run support for Clemens this year. 2.87 + 1.75 = 4.62. If you use 2.00 runs, you're still looking at 4.87.
Good news if you like the TT is Houston L4 games they scored over 5, but the previous 11 games they did not!
Not saying that Houston won't score O5 runs, just saying why the number is at 5.
