Seats Disappearing and Reappearing

I did some searching on the forums and didn't see anything about this that I could find.

Just started my airline, and getting ready to start my first flights. I've been doing some last moment checking, and I ran across this little oddity. I have a 737-400 BGW flying back and forth between two airports. On the flight from my hub to the other airport, there's 158 seats (economy and business) going out and 135 coming back. The specs show the 400BGW able to take 170 passengers. It's not a custom seating arrangement, and the weight payload is at about 73% of maximum. I'm at a loss. Why would the plane be carrying less than 170 passengers in total, and why would it change back and forth on each flight?

You explained it by yourself:

weight payload is at about 73% of maximum

Maybe you can take a look at the performance tool (detail page of the 737-400BGW) and there you can see why the payload is restricted - I guess it is the runway length at one of the airports.

I had checked the performance tool, but it seems you are right. The max needed runway length is about 10 meters short at one airport.