Why don't littleneck, cherrystones, razor and Manila clams typically require purging?
HOW TO CLEAN SOFT-SHELL CLAMS
Some clams, such as steamers and savoury clams, require "purging" — or soaking — to remove naturally occurring sand. I've heard of many tricks, but the most effective method is to make a very light saltwater brine (about 250 mL/1 cup non-iodized salt mixed into 4 L/16 cups cold water), and then to immerse the clams in it, keeping them chilled. You will need to change the water a couple of times — always using salt water — and it will take about a day of purging to get rid of the bulk of the sand. Littleneck clams, chertystones, razor clams, and Manila clams typically don't require this step.
For more detail, please see the sentence beside the red star.