I can understand Shanghai dialect, but Shanghai downtown can't understand it. Why?

In fact, the real Shanghainese is the local Shanghainese, and the so-called Shanghainese spoken in urban areas evolved from some dialects in Jiangsu, Zhejiang, Shanghai and Anhui provinces. As we all know, Shanghai used to be an immigrant city, so 90% of Shanghainese living in urban areas are from Jiangsu, Zhejiang, Shanghai and Anhui provinces. Therefore, the pronunciation and tone of some words in Shanghai dialect in Shanghai downtown are different from those in Shanghai dialect, so people in Shanghai downtown may not fully understand the local dialect.