I think part of the issue is what is defined by IGE or OGE, and the FAA doesn't make any hard and fast rules on this.
The numbers in the FM are based on the following:
for IGE, for Part 27 helicopters anyway (or any that aren't Category A), the IGE height given is the minimum height you can hover at, and do the takeoff as described in the manual (or if nothing is stated in terms of 'hover power +5%, for example, then using a fixed collective takeoff). As you accelerate through translational lift, having started from this height, you won't smite the ground. THis is the reason for the difference in hover height in the charts for example between high skid gear and low skid gear.
For OGE, it's the hover height the manufacturer states, and this is probably done solely for reference purposes. There is at least one helicopter that has a Hover OGE chart that you can get to if you start from that height by decelerating from forward flight, but can't climb to from the ground if you are at the power limits stated in the FM (i.e. it takes 100% to hover OGE at 80 feet, but if you start from the ground, using 100% torque, you'll only get to 45', because you haven't got enough power to climb vertically...)
Hope this helps the confusion (Nick -notice no mention of pressure under the disk...)