I've got a step brother in the topsoil biz, and we've discussed this question before...
The makeup of a 'perfect' topsoil mix would be exactly 1/3 sand, 1/3 clay and 1/3 silt.
And of that 33.33% 'silt' component; a certain percentage would of course be of organic origin- 50% or so, give or take a little.
Are you going to be able to buy such a perfect mix such as this all the time?
They're usually much more heavy on the sand and silt side.
This is because many topsoil outfits are run by the same folks that handle gravel pits and the like... and often they 'mine' the lowland of topsoil in much the same way as a gravel pit.