Sigma from Ppk is calculated using all of the individual measurements.
Sigma for Cpk is calculated using the variation of the subgroup totals. RbarD2
Since there is more variation in the individual measurements the subgroup totals, Ppk will usually be lower that Cpk.
That is why Ppk is a preffered methododology to use during initial capability analysis.
Generally I see <10% difference between the two.