Soil organic material by mass and by volume
There’s a big difference, with more than 25% OM by volume at just 5% OM by mass
Soil organic matter, and the total organic material measured by OM246 tests, are reported on a mass basis. That is, mass of organic matter per mass of soil. For convenience, this is usually reported as a percentage (%).
By using fixed values for the bulk density of soil mineral matter, and for the soil organic matter, one can measure the organic matter on a mass basis and estimate it on a volume basis.
The organic matter by volume goes up a lot faster than it does by mass, of course, because soil—and sand specifically in many high maintenance rootzones—has a higher bulk density than does organic matter.
This is another of the many things you can look at, at specific depths in the soil, when doing OM246 tests.