Over the past 500 years typeface weight naming has changed, with crossover from German, Italian, and British type designers. Roman, Italic, bold, fat, black, condensed, etc. were all descriptive names that were not systematized. One typeface’s bold weight could be equivalent to another’s Extra Black.
In 1957 Adrian Frutiger released Univers, meant to be used in all European languages. In Univers, light roman weight is designated “45”, normal “55”, bold “65”, black “75”, and extra black “85”. Eventually, with the release of Linotype’s Helvetiva Neue, the weight numbers were extended to three digits and became part of the PostScript font descriptor.
The names, however, remained in common use, and any type family which has weights heavy enough will have 800 Black and 900Extra Black. Potentially thousands of typefaces have those descriptors. Google neither invented nor systematized naming conventions. Also, most typefaces available from Google were not designed by Google designers. Google merely provides access to typefaces distributed by designers under certain licensing conditions.