Question for the 41 gurus. Today I was doing 2x20 threshold intervals. The 2nd felt much harder than the first. The 2nd interval also had a much lower heart rate than the first, but the heart rate for both intervals was always in Zone 4 (my LTHT is 172 - first interval was mid-upper Zone 4, while second interval was low Zone 4), yet both intervals ended up with the same avg speed and distance covered. I don't understand how that can be.

Here's the ride
http://app.strava.com/rides/10682708

Laps 2 and 4 are where the intervals are. I felt pretty good during the first interval and pushed hard. After recovering for 20 minutes, I slowly ramped up the pace again to just under Zone 4 in a little under 10 minutes, then started the 20 minute threshold interval. I felt like I wasn't going quite as fast but honestly wasn't all that focused on speed. I was just trying to go as hard as I could for the duration of the interval.

But when I look at the avg speed and the mileage covered for both intervals, they're essentially the same. So my questions are, why did the 2nd interval seem much harder than the first, why did I have a lower heart rate even though it felt harder, and yet somehow had the same avg speed/distance covered? I'm not sure how this could be.