Questions on the assignment for pandas

Hi,

Question 1:
Why did i get so many NaN when i used the first method (through utilizing the columns parameter) to create the df? No issue with the 2nd method.

Question 2:
Why did the name of the columns for sales_products_data changed as well if we used the 2nd method to create the df?

df = pd.DataFrame(sales_products_data)
df.columns = new_column_names