I'm not sure the details of your treatment process, but we used to have wildly different phosphorus levels from week to week, even day to day. We have a biological treatment plant to treat our industrial waste (chemical plant).

With the treatment process, phosphorus levels heavily depends on what is being sent with the influent waste stream and (in the case of biological treatment), the population and health of the microorganisms. I found the best way to keep the levels in check was to monitor them every other day through the use of Chemetrics tests (not good for reporting levels to regulatory agencies, but good enough to make sure the levels are monitored closely and adjusted accordingly).  

If you do biological treatment, let me know and I can offer more advice. If not though, I wish you luck!

'How can it be so low one week and then high the next?'

There is no large variability in your values. I would say they are usually low.

Suspended solids was a good hint.
What is the precision of the analytical determination?
Having an equalization tank should further make the effluent quality even more stable.

Kind regards
T. Sobisch