I mad this quick little chart to illustrate my point.
Someone could be making a killing working the AH. The gold per hour is great at first but as the day goes on and you have picked all the easy profits, you have to work harder to make more gold. The real profits were made when you first started.
Another person could just farm constantly. The gold per hour is no place close to that person working the AH when they start. But as time goes on there is a point where the farmer is making more gold in the same time as the person camping the AH.
You can make the best out of this by starting with the items that make the most gold per hour and knowing when to switch to something else. You may feel like you are doing nothing and making gold while you camp the AH but if you went out and did something your profits would greatly increase.