Think about winding the altimeter, if you wind the pressure down the altimeter goes down (try it in an aircraft if you are not sure)so:-
Sitting at 3,000' with 1030 set because that was QNH when you left, get further down route and the pressure is 1020 - wind it off and the altimeter goes DOWN by 300' so your actual height is 2,700', with a hill at 2,500' you definitely want to be looking out below!