Your confusion is quite understandable Gary, as it's hard to put a finger on fully . I won't endeavor to give too specific of examples, but suffice to say that while we here in the States can immediately recognize the accentual differences of say the deep South, New Orleans (Cajun drawl), the Appalachians or Ozarks, the Bronx and Jersey City, Chicago (twang), the northern border states (Canadianish), New England-Boston, they hardly constitute significant dialectal differences (with a few, relatively minor exceptions). In Germany, especially traveling its span east to west or north to south, these differences are much more pronounced, and much more numerous. They can be accentual, dialectal and/or idiomatic. From one end of the country to the other, depending on just what's being spoken, folks cannot always make out what's being said clearly. Then throw in the Deutsch of say the Swiss, Austrians or Alsatians, and you're muddying the waters even more.
By comparison to German standards, it could be said we ALL speak plain Jane English (ok, King's English, with nasal emphasis) here in the US, and with only relatively minor accent differences. The differences in Germany are more analogous to us as Americans hearing English as it's spoken in England, Ireland, Wales, Scotland or Australia, with the variations and idiomatic changes you'd find there, and between each (some clearly sounding more foreign than others to our Yankee ears).
Let's not forget also that English used to be German (Anglo-Saxon Deutsch), and still is at it's core! So did French (originally Frankish-Goth) and Spanish (likewise from the Visigoths), before they evolved into Romance languages upon fusing with Roman/Latin. And we're not even touching on the German that today is called Dutch or Flemish! Linguists and philologists also include the Scandinavian tongues (Norwegian-Swedish-Danish-Icelandic) as being Germanic (northern Germanic).
The reasons for the greater dialectal variations amongst the Deutsch stems back to the nature of the Germans existing (co-existing) for as long as they did as separate tribal clans, all with customs and cultures only partly shared, which were maintained for so many generations right up until Bismarck (and the Prussians) ultimately forged a national union, shortly after the Franco-Prussian War.
It's worth noting that dating back even to their arrival in Europe some 2000 + years ago, there were already in place certain dialectal and idiomatic differences between many of the tribes (more so with some than others), as noted by a few Roman historians of the day. This only magnified with their settling deeper onto the continent itself, and coming into more direct contact with the Greco-Roman world and with their distant cousins, the Celtae and Galli (Celts and Gauls). That each surviving tribal clan, as it evolved into a state or region of its own, also remained fairly isolated (before national unification only a century ago) no doubt further heightened this phenomenon.
Jb