It could be the load you run. Most boat trailers that I deal with run at 90% or more rated capacity. For example, tomorrow I am going 400 miles to Knoxville to bring back a 9,500# boat/trailer plus or minus. Its sitting on 2 5,200# axles. My other boat/trailer was 6,800# sitting on 2 3,500# axles. My other boat/trailer is 15,600# sitting on 3 6,000# axles. As you can see these trailers are all running near max load all the time. Same thing with most campers. You will find they are right up there also.
Now take a car hauler, GN, or live stock trailer. Yes you can load it up to max or more but most times they are running lighter than max loads. I would also venture to guess 1/2 them miles are with no load other than the weight of the trailer. For example, I drove 100 miles a few weeks ago empty to haul a old 8N back for a co worker so on a 200 mile trip my car hauler had just the trailers weight then the return trip about 3,500# worth of tractor but was still 2,000# below the max weight.
I am glad you have had good luck but most have not. Like I said, a quick search of google will tell you a lot.
Chris