My first guess is that this has to do with how gateways manage the availability of answers for students. The issue is whether they should be available at a fixed time, or at a time relative to the end of a given test version. For better or worse, the current design is to use the latter. This is described in the wiki, but the following is I think the same information.
When setting up a test, the open and due dates determine the time frame in which students are allowed to start a test. Then, when a student takes a version of the test, a version of the test is created for her/him. For that set version, the open date is set to the time at which the test is started, and the due date to that time plus the time limit for the test. Then, the answers available date is set to the due date plus the difference between the due and answer dates for the global set.
The argument for this behavior is predicated on the assumption that students might be taking multiple versions of the test at different times scattered over a long testing period. The argument for just having the answer date set to whatever the global set specifies is that it avoids the type of confusion that you've voiced. It would be a relatively easy change to make, though it would be nice to have a sense of how many people might be adversely affected. Perhaps the best solution would be to have a parameter in the configuration files that would allow selection of which behavior to exhibit.
Does that sound like what you were seeing?
Thanks,
Gavin