Not physiological -- just mental. You've hit on what keeps many a powerful cyclist from excelling in time trials and triathlon bike legs.

Holding a big gear and high effort level on the flats takes focused training. You should dedicate some workouts to simply holding high efforts on the flats. It takes practice and focus. You'll be much stronger if you do it. It happens to be my weakness, and I'll eventually get around to working on it.

I'm assuming you're trying to bust your gut during that one hour ride? ie You're not just out there to build base cause in which case the HR's should be more like 120's.

I found the opposite one time...punching into a wind I felt like death and yet when I turned around and felt like I was flying downwind my HR was actually about 10 beats higher. I'm no expert on the human body but I'm thinking when climbing you're taxing the muscles more whereas when you're on the flats you're hammering the cardio system more. Not sure how correct this is but the heart may act in different ways depending on what system is being taxed the most. Perhaps it's your lungs holding you back on the flats from being able to hammer your legs as hard. In that case perhaps more base work is in order.

Now I'll hand over to the physiologists to cut my theories to threads!!

Certainly while climbing, especially if you stand here and there you will tax your system more and reach a high heart rate. The more you train like this you will loose some of that ability as it's always harder to stand but there is some "muscle memory" and efficiancies gained after training. You will certainly be able to replicate while seated by going faster in a bigger gear or another trick would be to spin the current or smaller gear at a higher cadence. This will also cause a spike in heart rate even though your speed doesn't change. Lance does this often in training as one does develop certain economies at a given speed and RPM. His training also detected that this higher rate of pedaling eases recovery as the muscles take less strain. Now if you want to bleed from all your pores and cry like a suffocated banzai kitten, try cyclo-cross. I've done the series here in MN and although I'm not so sure it will benefit a long TT it certainly pegs your heart rate for an hour. I'm 39 and have a 185 for my max hear rate and spend the whole hour at 175 to 180. I do the A race in the 35+ class and get just about dead last. The real heart knocker is jumping on and off the bike and all the pace changes. I actually did alot of hills out of the saddle to prepare for this odd anti time trial position and it certainly has helped. If nothing else, it gives you reason to train as life gets bleak when the thermometer has a hard time hitting 32f.