1mb (or 1hPa as we now call it in Oz) change in pressure = 30ft change in altitude
1027.5 - 998mb = 29.5mb pressure difference
29.5 x 30ft/mb = 885ft
It helps to think of it as pressure surfaces & not altitudes. You can ignore aerodrome elevation for this as it's all about differences, so I'll say you're at MSL. What's the pressure on the ground at departure? 1027.5mb.
Given that pressure decreases at 1mb per 30ft increase in height, where is the pressure 998mb? Way above your head. 885ft above to be precise.
Climb to 885ft & measured the pressure - it will be 998mb. So if you flew at a constant 998mb pressure to a location where the QNH was 998mb what would be your altitude? Zero.
The pressure surfaces slope downwards to the lower pressure, so as you fly towards an area of lower pressure your height above the ground decreases. Remeber you're flying at a constant pressure, not constant height AMSL. Which means your altimeter is reading high as you fly towards the lower pressure.
I think of it as the low pressure is above you it comes down to meet the ground as the QNH decreases - as the pressure descends so do you.