I'm guessing it's because they represent different objects - the "parent" object in AAD and the workload-specific representation in ExO. In particular, if you have some very old ones, you might see that they all have a very similar creation dates in ExO, corresponding to the time around which Microsoft changed their recipient type, while the AAD value is a very different one.
thanks, so what is the difference between that one and Get-AzureADGroup?
I found a note stating that "Going forward, all cmdlets that call Microsoft Graph will have “MS” in their cmdlet names, as in “Get-AzureADMSGroup”. The cmdlets that call Azure AD Graph will not change, so there is also a “Get-AzureADGroup” cmdlet."