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.
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.