They set it at $159K and now brought other realtors from the office through and changed it to $153K. Why did they not set it at $153K to begin with?

The Realtor cannot change the price without your permission. It is your house. If you did not authorize the price reduction then they should not have changed it.

As far as the reasoning, the one Realtor who listed it probably thought $159k sounded like a good price and listed it at that price. That is what should be on the contract that you signed. After they did their property tour they talked about the houses they toured and the general consensus may have been that the house might sell faster at $153k. But, like I said earlier, they don't have the authority to change the price without your permission.

Did the Realtor call you and tell you the price was changed or did you see it advertised somewhere? I just wonder if there could have been a typo in the advertising.