Hi Mike, maybe i can clear things or help you as i am pretty much the same as you (age, pbs, hr). I was wondering the same thing because i also experienced this. My easy Zone according to 5k pb should be 4.30-4:00/km but in reality i often had to go 5-5:15 to feel truely easy. Hr ~130 is pretty much impossible unless i go 5:30 or slower. At 4:40 im often at 150-155 (max is 203). I took a little bit of a deep dive into my training, normal day, eating, talked to some experienced coaches and this is what found:
-some people (like us) rely heavily on their VO2 and/or speed reserve to generate performance or the current LT, while others generate it via big aerobic base and high easy pace
I myself didnt realise this until a coach told me. I thought of myself as a full on endurance type for years, despite is was a decent longjumper as a kid and my best running distances being 800/1000m...
Some other indicators could be that you also dont like long tempos, continuous slow runs and love intervals paced at vo2 or faster. Eg i get easily fatigued at least more than i should when runnnig longer than 45 steady, my hr creeps up relatively fast between 150-160 for the pace. Some days its almost Z1 according to pace while hr is almost entering Z3... even when training at LT2 my hr goes to 180-185 quickly. When you look at the pace it seems slow, but between 180 and max there is a surprisingly huge reserve in pace i can generate. Maybe this is true for you also?
Also do your performances get slower with the distance going up? i suspect so. For true endurance types all this is the opposite- they have big reserve in the low end hr zones with fast basic paces up until LT and then hit the rev limiter relative quickly.
-second i found that i have very unregular lifestyle, unregular bedtimes, training times, often switching days, doing other random activities like hikes on the side, eating whatever. This effects hr also. When i was in training camp i had the same day always and my hr was much more predictable, still on the higher side but not 138 one day and 155 the next...
Alcohol is a big one... 1-2 beer -> +10bpm next day for me...
-higher miles and running every day also affected my hr. I think i recover but was not for whatever reason. Test this, for a week run easy pace but only every 2nd day and look at the hr. When i did this it dropped and matched my pace, meaning i was not recovered before... which brings me to point 1 again and that i have to run exceptionally slow every day to be recovered ( i run easy now with people almost 5min slower in the 5k than me).
-not eating enough the day before, makes also almost 10bpm plus for me...
Hope this helps a bit. My personal opinion is, i suspect you under point 1 as a more speed oriented guy. Maybe your training is also more focused on intensity and therfore the lower hr zones dont match the paces.