I believe you are right the sub soils usually are more dense because of the eluviation of fine clay but I have seen that in volcanic areas there are many times depositions of pumice materials which give a lower bulk density at a particular layer. Generally the olders soils have clayey and compacted subsoil layers but this is variable depending on the soil formation conditions and is modified by human management practices. Increasing soil organic matter is a method of reducing soil bulk density.

I believe you are right the sub soils usually are more dense because of the eluviation of fine clay but I have seen that in volcanic areas there are many times depositions of pumice materials which give a lower bulk density at a particular layer. Generally the olders soils have clayey and compacted subsoil layers but this is variable depending on the soil formation conditions and is modified by human management practices. Increasing soil organic matter is a method of reducing soil bulk density.

By and large bulk density is a function of soil particle density ,soil compaction, ,porosity and organic carbon .The BD of arable soils is lower on surface layers and progressively increasing with soil depth unless encountered with intermittant porous layers. Lower bulk density leads to better aeration, soil aggregation ,more hydraulic conductivity, better nutrient uptake by crops and greater crop yields.

good question

Would you please detail your observations by providing soil type and its US taxonomic class?

Generally bulk density increases with increase in depth. But if the soil has more porosity then it may decrease. If the soil has more organic matter then soil microbial properties will be more, soil aeration will be more so there is a possibility that BD may decrease for better results.