This is because update-manager actually performs an commit operation, not an upgrade one, to allow selection of individual packages to be upgraded. However, I don't know if software-center would handle this even if update-manager was fixed, so leaving both tasks open.
This is because update-manager actually performs an commit operation, not an upgrade one, to allow selection of individual packages to be upgraded. However, I don't know if software-center would handle this even if update-manager was fixed, so leaving both tasks open.