It's working correctly. The BBC usually only put active content in the 14:9 region, and this is what the STB is showing you. (It is maximising the useful picture area). The older Pace boxes aren't capable of doing this so gave you the inactive bits of the picture, together with more black.
Take a look at page 22 of
http://www.dtg.org.uk/publications/books/afd.pdf. The Samsung box is doing what is shown in the top line. The Pace box has to make do with the second line.
You
may be able to work round it by setting the STB's output to 16:9. Or this may just give you a distorted picture. It depends on your TV.