|MadSci Network: Microbiology|
That's an interesting experiment, and an interesting question. To answer the question, however, you would have to do another experiment using distilled water and tap water. If there was no inhibition from distilled while there was from tap, it would have to be one of the substances that tap water has that distilled doesn't.
Can you think of anything that the water company might add to water that's delivered through plumbing as opposed to water that's delivered in a sealed bottle?
Try the links in the MadSci Library for more information on Microbiology.