Originally Posted by Yachtzee
I believe that tendency comes from the influence of German on American English and is common in the Midwest, where large populations of German immigrants influenced the vernacular. In German, some words are the same in adjective and adverb form. The word for "real" in German is "echt." However, "echt" can also mean "really" depending on context. So a German would say "Das ist echt cool" (yes, many young Germans say "cool") or "Das ist echt Gross" and it would translate as "That is real/really cool" or "That is real/really big." Another example would be "langsam," meaning "slow/slowly." A German would say " Bitte, langsam fahren" to mean "please, drive slow/slowly." As the German lacks a distinct adverb form, German immigrants likely used the adjective form for both.
Interesting. I actually took German, so I recall what you are referring to.