It could have something to do with the underlying surface. Asphalt is considered flexible, whereas concrete is rigid. The ramp is a multi-layered structure, that is built to ensure that applied loads will not lead to distress (term is preferred to failure in this subject) in it or the lower layers.
Here in the US the accepted standard is the CBR
Procedure adapter to airfield pavements. CBR
method was developed by the California Division of Highways back in 1928. This was then adapted by the US Army Corps of Engineers after the outbreak of WWII.
The test expresses an index of shearing strength of the soil. The test consists of compacting about 10 lbs of soil in a 6-inch diameter mold, placing a surcharge on the surface of the sample, immersing the sample in water for for days and penetrating the sample with a steel piston approx. 2-inches in diameter at a specified rate of loading. The resistance of the soil to penetration, expressed as a percentage of the resistance for a standard crushed stone, is the CBR
of the soil.
It appears that is a matter of apples and oranges.
(reference:Planning & Design of Airports, 3rd Edition, Robert Horonjeff/Francis X McKelvey, McGraw Hill 1983)