I think given timing (it's September and people are going back to school), they lowered ticket prices for certain weeks. Granted, I don't think it is enough to make a huge impact on people's buying decisions, but I think that is why it is lower those weeks. I was under the impression that the base ticket price was $137.50, and with this special 50% off discount, they did not change the base price as they did for other weeks.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611