That's normally the way it is, Tx (tramsit) from something goes into Rx (receive) or something else.Normally should be MCU_Tx - GSM_URx & MCU_Rx - GSM_UTx
But if you are receiving the "OK" which only the module is sending then, however it is connected, would seem to be correct.
It could be down to how the module manufacturer has labelled their carrier PCB rather than than the way the SIM900 unit itself would have it.