A MU with variables is exported via uxext, but when importing it via uxins, the MU variables are not imported.
On the other hand, when Sending the MU to a Package or xml ( with uniexp), the MU Variables are exported.

This is expected behavior, MU variables are a new Object introduced on version 6 and uxext / uxins tools are kept for compatibility reason from version 5.
Since they did not exist in version 5, the uxext command does not export them.

Dollar Universe 6

On version 6, use the uniexp tool available in UVMS to export to XML the Management Units with all its content ( Dependencies / Variables / Timezone / Directories) and  the command uniimp to import the Management Unit.
Else, the Management Unit  can be sent to a Package where they will also keep the same Definition upon Deployment.