Why one Manuka honey is stronger than another
The strength of a Manuka honey is decided in the nectar, before a bee touches it. That is why two jars from the same country, the same year and the same producer can carry very different numbers.
The number is a compound, and it forms slowly
The MGO figure on a label is milligrams of methylglyoxal per kilogram of honey. Methylglyoxal is not made by the bee. It forms non-enzymatically from dihydroxyacetone, a compound present in Leptospermum nectar, and the honey's non-peroxide activity correlates strongly with how much methylglyoxal it contains (Cokcetin et al., PLoS One, 2016, PMID 28030589).
So the ceiling is set by the plant. If the nectar carried little dihydroxyacetone, no amount of careful handling will produce a high-MGO honey.
Species, and the size of the range
This is where the variation comes from, and the range is much wider than most people expect.
A survey of nectar from fifty-five of the eighty-four Australian Leptospermum species found dihydroxyacetone from undetectable in some species to more than 16,000 mg/kg relative to total sugar in Leptospermum speciosum and Leptospermum whitei (Williams et al., Journal of Agricultural and Food Chemistry, 2018, PMID 30289260).
Undetectable to sixteen thousand. Within one genus. A hive worked on the wrong species in the wrong week produces honey that is perfectly good honey and will never carry a meaningful grade.
Place matters too
In the same 2016 study of eighty Australian Leptospermum honeys, the highest readings came from particular places: Leptospermum polygalifolium honeys from the Northern Rivers region of New South Wales and from Byfield in Queensland reached up to 1,100 mg/kg.
Which is why serious producers do not simply buy Manuka. They know the stand of trees, the species and the window when it flowers.
How well it keeps
One of the more useful findings in that study is about storage. Activity was stable over seven years of dark storage at four degrees.
Methylglyoxal continues to form from dihydroxyacetone over time rather than degrading away, which is unlike most things in a food that carry an activity number. A well-stored jar does not quietly lose what you paid for.
What to do with this
Treat the grade as a measurement of one specific compound, not as a general quality score. A honey with no grade is not necessarily poor honey; it may simply be from a species that never had the precursor.
And treat a number with no test behind it as an adjective. The next article covers what a certified grade actually proves.