Aneroid Barometer

The Barometer readings have been falling back over the last couple of days as Storm Babet approached and it is still falling slowly. This Barometer was a wedding gift to my parents in 1939 and still continues to work accurately although calibrated in inches. A more modern version giving the same pressure reading but in millibars has the spear pointing at "Stormy". I wonder why there should be a difference A mostly cloudy day with sharp showers, but very mild.