If by "projecting directly into the astral" you mean phasing vs. having a separation into the RTZ, there are various factors that have to do with it- most phasers are visual and have very developed clairvoyance, while I believe that most 'natural' OBErs tend to be tactile, and tend to clairsentience.
Other than that, I do think most people project into the RTZ first and may not realize it, and others project into the RTZ and don't stay there at all, or immediately shift as soon as they've gotten their bearings.
To add to this, sometimes an astral projection seems like an RTZ projection until things start to get weird fast- this is an indication that you've projected into a simulation (buffer zone between the astral and RTZ) or astral that looks like the RTZ.
So it's not so simple.