The chair legs are not manufactured correctly. There were only 4 chairs out of 8 that did not wobble. This is after tightening screws only 80%, checking for the wobble, and then tightening them to 100%. There is a fundamental flaw in the angle at the base of the leg which attaches to the lower side of the chair base. Due to this the legs come out at different angles and produce the wobble. On 1 chair the misalignment was 7mm, which is very high for a chair this size. 

It seems the 8 chairs came in 2 packages which differed in quality. One package of 4 chairs had the wobbling problem, no long screws for the chair back, as in the 2nd package, the leg-caps came pack separately. The two packages just seemed different, and the chairs from one package all wobbled.