It's possible that Bournemouth will win Chelsea because of their consistency in their previous matches. Chelsea's performance is not stable and unpredictable which makes it impossible for you to rely on them. Bournemouth is in their best form and shouldn't be underestimated because they don't have a big name. Chelsea needs to improve their performance if they don't want to be bonus point for Bournemouth.
Chelsea are experiencing a decline in performance lately, where in the previous 4 matches they have not won even once, so it is clear that the match when hosting Bournemouth later will not be an easy match for them and also considering Bournemouth are in a positive phase this season so I think even though Chelsea chances of winning are higher, it does not rule out the possibility of losing again.
Thus the safest bet in their match is BTTS.