It seems that some people are not aware of this:
Bi-specialization effort has been started since 04/2006 and it will be realized in several steps.
Gradually, VSC/CSC will stop dealing with I-140 but specializes in non-immigration applications (H1 and so on), and TSC/NSC deal with all I-140 and its corresponding I-485/AP/EAD.
This is why from Apr 2006, all new I-140 application and their corresponding I-485s are all ted to NSC (half were rerouted to TSC). A lot of my friends' cases were transferred from VSC to NSC at that time. But of course VSC/CSC still have some leftover cases (such as your guys' cases) to deal with.
But I guess USCIS does want to clean up the leftover as soon as possible to finish this transform.
So, your cases were transferred not because TSC/NSC has fewer cases than VSC/CSC. It is not possible because VSC/CSC do not accept new applications since 04/2006 and a lot of cases have already been transferred out. Your cases are transferred to TSC/NSC just because USCIS want to expedite the bi-specialization process. And it seems to be a reasonable move.