I hadn't noticed myself, but apparently there are quality issues with BBC HD that have led to them issuing the following statement:
"The BBC has deployed a new set of HD encoders. The current bitrates were picked after much side-by-side comparison of the old and new encoders using a wide range of programme material. While it is impossible to achieve parity in all formats the new encoders were judged to be giving equal or better quality in most cases."
"However, there is no test like putting these to air, and when we did, a reproducible problem was identified on certain types of material. We have shared this information with the coder manufacturer so they can investigate further. "
"The particular problem does not go away with higher bitrates, so whilst an investigation and fix are awaited some changes have already been made to the encoders to try and mitigate the effects. In addition, the new encoders also fixed other problems, unrelated to picture quality, which the BBC received complaints about."
There's more on this on the What HiFi site.