Why are currencies not = to its inverse? (SGDAUD != 1/AUDSGD)?

We consolidate our currencies across multiple vendors, and the data is coming directly from market participants and/or contributors. As such, for most rates (depending on the rate, and availability), we will not be returning/calculating inverses, instead returning the real market rate, which may not = inverse of same rate. Additionally, there is the matter of timing as well, since even a slight difference in timing for the realtime endpoints can result in very different values.