Heart rate during olympic

I did Chicago last weekend, my first Olympic tri.  Recently, it has been getting more and more difficult to get my HR up into target range (the calculators always put this at 140-160).  I figured that this is a good thing and probably I was getting into better cardio shape.

Going into the race, my goal was to focus on keeping my HR around 150 (the middle of my range) and not pay any attention to speed.  A mile or two into the bike though ,I realized my HR was up at 175. I backed off and brought it down into the mid 160's.  I kept monitoring for signs that I was wearing out but they never came.  I continued to keep it in the 160's but as I sped up a little toward the end the run (I was feeling good), it crept up.  I finished the race with an average of 163 (the swim brought the average down).

Why didn't I bonk?  Adrenalin?  Is this what I should expect going forward?  Maybe I have just miscalculated my ranges? I guess I can probably be training a lot harder than I have been.  I know there are those that will say that this is why HRMs are unnecessary.  I am still curious though.