Also the Tiagra 12-28 is not the same cassette as the 105 12-27. It is missing the 16t. This seems to be intentional, the message Shimano is sending is if you want high performance gearing, you have to pay for it and upgrade.

12-13-14-15-17-19-21-23-25-28
12-13-14-15-16-17-19-21-24-27