One of the attributes of a good bonsai soil is that it remains moist but does not become water logged.  Pure akadama retains more water than many bonsai would prefer.  Neither pumice nor lava retains much water, so mixing akadama with pumice and lava gives a soil that retains a reasonable amount of water, but does not become waterlogged.  Again, sadly, there is no firm data to support this speculation.

Water retention is only a part of the story.  The other huge side of the equation is how frequently the tree is watered.  Collected junipers can be not only established, but grow well in pure pumice, which dries out relatively quickly.  They just get watered a bit more.  So what is the bottom line?

The popular soil mix of equal parts lava, akadama and pumice works well.  The use of lava and pumice reduces both the cost and weight of the mix. The most important criteria for success with your bonsai is to adjust the frequency of watering of whatever  mix you use to your particular conditions so the soil is moist but not waterlogged.