Bivalve vs Gastropod

[Marlo9]

Keep this in mind.  For every "bivalve"  there are two valves to wash up on the beach.  So, your ratio is really 2:4, not 1:4. 

Bivalves are lighter and flat and rise far more easily in roiling water, thereby being more likely to make it to the shore.  They also tend not to roll back down the shore to the water.

Far more univalves are "micro" size and are not readily visible on the beach (or have been so rolled around on the way that they are not recognized as shells.

Most of all, univalves are more highly prized and all the other collectors got there before you and took them home.