The Sunny 16 rule is an approximation only, at best, an estimation which must be used with experience.
BTW, is the meter set to 0.3 or 0.5 stop reading in its custom functions? Customised readout is one of the biggest causes of confusion with Sekonics because there is no visible warning on the display that a core metric has been altered, other than to realise the exposure steps at wider or narrower. Having said that, it is most unlikely that the Sekonic is at error with its metering: it is very fine and precise instrument. People really should move on from these cute, clubby, "rules of thumb" that only provide for increasing risk and failure and not refinement of metering technique that is so much a calling when armed with a Sekonic.