Time of Day, Temperature, or Combo?!?!?!

Probably a combination of things, but a 20 degree difference in outside temps makes a big difference in your body's ability to cool itself and not need to keep the HR up to maintain blood flow to the surface for cooling purposes.  Time of day seems a lot less likely overall.  But your attitude that day may also affect things.  I find a lot of times when I tell myself I am going to have an easy slow run, for short runs, my time is actually better, and I have more reserves than if I plan to push the pace. Weird.

My other thought is not to get too caught up in the numbers game.  Pay attention to how you are feeling during the run, glance at your HR, and see what numbers reliably correlate to feeling that you are pushing yourself. I usually run an average HR of around 160 for runs, with peaks in the low 170's. So I know I am "cheating" if I back off or walk with a HR of say 164, since I can reliably maintain a higher HR without problems.  On the other hand, high temps or humidity means my same route will give me a slower time with a higher overall HR, and more times I may need to slow down or even walk to drop into the 160s again. 

Buc,

Besides the HR...did it feel harder?  Did you feel exhausted, or at least much more tired than usual?  That has happned to me and I mainly contribute it to the heat but I think it I may have been a bit dehydrated as well...which made it seem even hotter.  SO you are not alone...I wouldn't worry about it.

 From reading a lot of posts here I am thinking about simplifying my runs a bit into the Easy, Moderate, Hard categories and giving my HR's a wider range.  I want to run more on feel than HR because of these differences.  It's also kind of fun to run and "guess" your HR and then look at your watch...I surprise myself how in tune I am sometimes.