You have no idea what team you will get from match to match - good showing vs Arsenal, then embarrassed by Brentford, then good matches against Spurs and City, then embarrassed by Newcastle, then a good win against Brighton, then terrible again vs United.
Its a young team but I also think its the tactics. All their best matches have involved them coming out like a house on fire, especially in midfield, and outhustling teams that are having trouble getting out of gear. This is how they went up against Arsenal, how they battled City, how they played against Spurs and eventually induced their players into some very rash behavior. But when Chelsea aren't winning the energy battle in that kind of high paced match, there isn't much to fall back on in terms of defensive organization, attacking patterns, etc.
This was also the formula for Poch at Spurs, except fewer other sides could match them in terms of energy and athleticism. And he had Harry Kane.