Sometimes you may notice different sites online list a different number of episodes for certain programs. For example, some listings of Downton Abbey show seven episodes in the first season, while there are currently only four episodes available at PBS.org. The episodes at PBS.org have combined content, so even though there are "fewer" episodes, the full season is included within those episodes.
This occurs because of how the programs are aired and uploaded online between the United Kingdom and the United States. On occasion, episodes broadcast on television or streamed online in the United States will combine content, leading to longer episodes in the US made up of episodes which were aired separately in the UK.
If you are looking at a site outside of PBS that lists a different number of episodes, you can also check to see if they list the episode runtime - the runtime of the PBS.org videos will be longer to reflect the videos are made up of combined episodes.