The bio-active property of Manuka honey is not static, but is ever-changing. The activity level increases from time of harvest for about 18-24 months. At that time, the bio-activity levels off at its maximum and over the next 2-3 
years the honey will start to drop in activity level. To give you some perspective, a honey that tests at a 10+ level at harvest might be a 14+ if held for a year after harvest. Many packers of manuka honey buy honey at harvest time and hold it for a year or more prior to packing it for sale. In this regard, Manuka honey resembles wines which are aged prior to bottling and sale. The rating that we put on our jar is the lastest test that we have for this particular batch of our honey. The actual activity of the honey at the time it reaches you is, in all likelihood, slightly higher than the rating on the jar.
We think of our honey as a gourmet food with the health benefits as an added bonus. Our experience producing honey in Taranaki is that it tends to test around a 10+ coming off the hive, and may grow to the 15+ range over time. Each batch will show some differences based on the actual sites from which the honey was harvested. We are currently utilizing about 50 sites spread across the Taranaki region.
Many companies selling manuka honey will buy it in bulk from all over New Zealand and blend it to achieve a certain activity rating. Typically their pricing will reflect that rating so you'll pay more depending on the bio-activity. There is a more general overview of testing methods & different rating scales under the About Us section of the web site.
Purchase Bees & Trees Manuka honey.
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