so booster seat safety web site

dionnemd10

i am looking at this site   https://www.nhtsa.gov/nhtsa_eou/info.jsp?type=booster

 and they reduce the safety rating of the seat if it comes unassembled-does anyone know why?   or at least they reduce the "securing the child" rating due to the unassembled product.   I don't understand the reasoning?

 I am looking at 4 different seats--harmony lite rider, evenflo big kid amp, cosco high rise, and graco turbo booster--anyone with experience on any of these?  thanks