No mistake...it is a lot simpler for individuals (lay or clergy) to become members of WA/LCMC than for congregations. Congregations must follow their existing constitutional requirements for meetings and vote plurality plus obtain approval of their judicatories. Many more hoops to jump through. Many laity are not aware that individual WA membership is available to them. The clergy are better informed, thus the ratio.
My thoughts were that some people may also be members of several different congregations. I know several people who attend and are “members” of more than one church.