If a jaundiced mind can put a few caveats into the discussion ..
(a) be very wary of published light aircraft performance data
(b) if it comes from the manufacturer, it will be based on just about the best conditions you can find
(c) an older aircraft, turbulence in particular, ISA plus deviations, non-optimum handling by the pilot .. etc ... etc.... and the quoted level goes down ... down ... down ...