As stated by PPrune Radar a Regional QNH is a forecast of the lowest QNH for an entire Altimeter Setting Region and is normally available up to two hours in advance. An airfield QNH is an actual QNH and at many airfields the QNH is updated every thirty minutes.
On a day with a shallow pressure gradient the two may be fairly close but on a day with a steep pressure gradient there can be a significant difference given the size of the ASRs. Also given the vagaries of forecasting the Regional QNH may be forecast incorrectly with a tendency to produce a lower value than the QNH anywhere within the ASR.