First off, just because Phosphate tested high (this is
water soluble phosphate) doesn't mean you are way out of whack plant growing wise.
Remember that soil tests are done on the premise of chemical fertilizers being used for artificial nutrient availability over those who use nature to grow plants.
What types of wood were installed in the Hugel? were they perhaps phosphate rich woods? It certainly seems that is the likely cause of the difference.
Many of the hard woods will have quite a bit of phosphate bound into the lignin and as the wood rots that mineral becomes free for uptake by microorganisms.
Well built hugels will have a good variety of bacteria,
fungi and other microorganisms living in the soil, that means there will be a bacteria that works on the wood to release the nutrients stored in the wood.
You also noted that you didn't see and deficiencies so congratulations you built a super hugel bed.
Redhawk