What is the difference between fromkeys and dict comprehension?

Question

What is the difference between the following 2 lines of code

dict_a=dict.fromkeys(['aaa', 'bbb'], pd.DataFrame()) 

and

dict_b={sheet: pd.DataFrame() for sheet in ['aaa', 'bbb']}

For both lines I can assign different changes to the dataframe of 'aaa' and 'bbb' individually like I want, so what is the difference between the two lines of code?