19mbit of MPEG 2 bitrate != 19mbit MPEG 4 Part 10 (h.264) bitrate.

OTA HD broadcasts use MPEG 2 compression. Most BluRay discs and most streaming/download services like iTunes use H.264. OTA is also capped at 1080i, BluRay and other services go to the full 1080p.

So direct comparisons aren't really easy here. Add in complications with source differences, and it becomes a really mixed bag. At the end of the day, it's possible for an iTunes download to beat out the quality of a BluRay or OTA broadcast just as easily as an OTA broadcast could put iTunes or BluRay to shame.