The big difference is that they finally can have winning streams and go unbeaten for several games in a row. This was fundamentally different last season when after every win there was a defeat inbound, which was a real roller coaster and once they thought they are on a good way, they got beaten by a team from the relegation zone. But now they seem to be up to things and know how to handle teams of all levels, which is good for the competition in the Premier League.
If you mean Chelsea, they have certainly surprised a lot of people this season, especially after fourteen games in the Premier League. I really didn't expect Chelsea to change like that this season where they have won a lot of games against the weaker teams and also won against the stronger teams in the Premier League. And Chelsea have only lost two of their fourteen Premier League games so far, which is really impressive because Chelsea are back to being able to compete at the top of the table this season.