Because the aircraft is experiencing port drift, it is heading into an area of higher pressure (wind blows clockwise round an anti-cyclone in the Northern Hemisphere). If going into higher pressure, the altimeter will under-read, ie the aircraft will be higher than indicated by the altimeter. This is safer than the converse: when entering a lower pressure area, the altimeter will over-read, ie the aircraft will be lower than indicated, or 'When flying from high to low, look out below'.