The garden "soil" you have looked at....is it a mixture of peatmoss, compost, dirt, or what? 10% of real mineral soil is mighty low...need more than that, but in a light mix, it is fine.

Good garden soil, classified as loam, is about 45 percent sand, 25 percent silt, 25 percent clay and 5 percent organic matter. That said you will find those that disagree on that statement.
Perhaps this episode of Growing a Greener World might be of some help.

Here is a link that might be useful: GGW 417

There's nothing wrong with a bit of sand in there, to answer your question. And the ratio of compost to soil is not critical, but if it's very low, obviously it may not be very fertile, and the texture of the soil fraction will dominate (so if they used a lot of clay, you'll know it immediately). If the compost content is very high, it will shrink a lot as it continues to decompose.

The soil I garden in here is about 92 to 98 percent sand depending on how diligent I am about getting organic matter in that soil. That sand, at the start, did not drain well at all because of high water table and hard pan. I cannot do much about the high water table but adding organic matter over the years has eliminated the hard pan and most of the time the soil drains quite well now.
10 percent sand in a soil mix is barely nothing and may not be enough, but many people garden quite well without any sand, just adequate amounts of organic matter in the soil they have.
Sand, or some large size particle (vermiculite or perlite) is added to clay soils to promote drainage, but it requires quite large quantities of sand and 10 percent is not nearly enough.