Hi. I was having the same issue ( about 15% more watts with the Core) with the same material than you. No way my left leg is that much weaker! And did all the calibration ceremony more than once. I just had the Core but was already suspicious about the 4iii because of unconsistent results (and because I had to glue back a small metal contact for the battery). I wanted to clear my mind so I ordered a new 4iii dual side this time in exchange of my actual left side one( they offer a big rebate for the exchange when out of warranty). So waiting my new PW and just hope it will be as generous as my Core with plenty of watts!