According to my RN73 datasheets, the Working voltage is guaranteed across the normal operating temperature range. For these devices, that's normally -55C to +125C (although they do state they have +150C devices avalable for special order).
The data sheet shows that for a 0805 device, there will be a surface temperature rise of 12C at 100% rated power.
The 0805s on this data sheet are rated at 100V max working, with an absolute max overload voltage of 200V.
That differs from your numbers a little, so it could be the construction (RN73 is thin film NiCr) and perhaps the level of detail in the data sheet.