By coincidence. Water happens to approach 1 g/cm³ = kg/l.
AFAIK baby birds (hardly pneumatized) have 8.5, while adult birds have 8.0. Something between these values might be more accurate for tyrannosaurs and produce a lower weight estimate and higher strength indicator.
not GN/m², which would be a pressure, while the inverse sounds strange?
Does anyone know the strength indicator of an elephant?