The kind of petri-dish (settle-plate) mold testing you refer to uses a small, flat dish containing a mold nutrient. The dish is supposed to be left open for a set period of time, then closed up and allowed to stand for several days, either in your home or, for an additional cost, in the lab to which you send the dish for analysis. Either you or the lab counts the colonies that grow.
The Money Pit's indoor air quality expert Jeff May doesn't trust the results of such tests, for a number of reasons. He notes that some people leave the dish open too long or run the test with the windows open, both of which can impact accuracy of the results.
Plus, the recent rapid increase in the number of consultants investigating mold problems and proliferation of laboratories providing analyses (and perhaps using unqualified technicians to do so) have led to many misleading reports.
May recommends that you next have mold testing done by professionals and the results analyzed by a qualified lab. If this second mold test confirms your original results, hire professionals to find and eradicate the sources of mold contamination under containment conditions.