I studied German in school (high school/college) and then moved to Germany when I was 37 years old. Although I knew I wasn't fluent in German, I felt confident in my language skills. We lived in a small village and the locals could understand me, but it seemed challenging for them. Finally, a neighbor told me I was speaking "Hochdeutsch" (high German, formal) instead of the region's dialect. Like your experience, they knew what I was saying, but it was way too formal and proper for the conversation. They would laugh before responding - I had no idea I was speaking so formally!