The astraes are the ones that cannot flip themselves back, I believe. The turbos are supposedly notorious for knocking things over and Trochus supposedly are long lived.
I am sure sometimes the LFS mixes up snails and sometimes one astrae snail is sold as a trochus and vice versa, for example. So you will end up with a Astrae that lives forever and a Trochus that cannot flip itself upright. It gets awefully confusing because then you start thinking Trochus'es die once flipped on back while Astraes live long, etc.
Usually, any dead snails are made of meals by hermits, other snails, cleaner crew, etc. You should not see much of a spike in your routine tests unless a whole horde of them die at once.