There are two types of converters, Isolated and non-Isolated. On the non-isolated ones the input negative terminal is the same as the output negative terminal, on the smaller units there is only one terminal and on the larger units there are two, but they are connected internally.
The isolated units have separate input and output negative terminals and they are not connected internally. There is a small price premium associated with this feature. I hope that explains it. This particular model is non-isolated and has a single shared negative terminal.