Heart rate zones are inconsistent

[MCOSR]

I've been using a heart rate monitor (Nike triax c6) for about 5 months. I programmed it for the fourth time today using the watch's zone finder (ride 5 min easy, 5 min medium, 3 min hard). It seems like the zones for the last two are all across the board and I am trying to determine if they are accurate or if something is wrong with the HRM. Below are the results from the 4 times I've done it.

Z1: 132-142
Z2: 142-176
Z3: 176-180

Z1: 129-147
Z2: 147-172
Z3: 172-180

Z1: 155-167
Z2: 167-173
Z3: 173-180

Z1: 144-149
Z2: 149-163
Z3: 163-169

The last two were done within about 30 days of each other. On today's ride I had about an hour between church & watching my kids while my wife went to a meeting so I was hustling to get on the road and might have started from a higher rate than usual. Also I'd already had 4 cups of coffee in the previous 3 hours. I ride ~300-350 miles per month in 20 to 50 mile individual rides (avg speed 18-20) with intervals once a week.

Not that I expect someone to have the answer based on this info. I guess I am just curious to see if this has happened to anyone else and if there is any reason for it. I don't really use the HRM for anything other than as another set of numbers to look at after a ride, so it doesn't much matter to me what the zones are. It just seems odd to me.

 

[Doctor Morbius]

Nobody here is going to derive their HR zones this way. If they're using HR (many have switched to using power meters instead) they're basing their zones off of their max heart rate (British Cycling, Ric Stern) or some percentage of a base line test performed (Friel, Coggan, Charmichael).