Why does boost jam build repeating libraries?

When I built boost::thread library I got to the stage library and two of the release and debug libraries were the same file size. One was supposed to be for dynamic linking (which for some reason didn't prefix with "lib") the other two were the same size but one had the prefix "lib" that the first didn't, and the last one had the s indicator for static linking. What is the difference between the ones that don't have static and those that do, but they both have the same file-size. Here's a picture, which also has the same thing for the regexp library.