Let me chime in here ,being 40 years in audio ,and owning a Audio store for almost 10 years. There is No standard on true specifications , if for instance the speaker gets to 28 hz for a fraction of a second at 90 db they will specify this .
playing bass to a constant SPL is the hardest thing to do ,bass starts falling off fast
you will not get a speaker That can do a honest 20khz to 20 hz
continuous unless you have $$ deep pockets . Having no Set standards there are many ways to stretch the truth . You want to see true graphs and waterfall plots
for a sustained time ,but almost never due , plan on spending at least $30 k on up
to companies you can trust.