There are several interesting points in this thread.
First of all, it should definitely be more difficult to raise your HR while indoors on a trainer. Your body is less affected by the physical environment. Also, there's something to be said about your level of awareness and sensory perception when riding outdoors, also making your HR easier to elevate.
The years of data that may be kept on record prior to training specific to LT, should be considered a completely different "ball of wax". Sure, you can push to a certain limit, and hold your performance there...but this has a few other factors to consider. Did you always maintain the same fuel storage and intake intervals, were you always properly hydrated, was your sleep schedule consistent, etc.
Perceived Exertion (PE)- this can also vary. A recent study done by JMU (which hasn't even been published yet, I think) found that certain amounts (relative to body weight) of caffeine can actually increase performance while the RPE does not change. Could this be due to vasodilation, or the analgesic properties of caffeine? I'm not sure. So, does your caffeine consumption always stay the same from day to day...and especially prior to, and during your rides? Stress levels, sleep, mood, overtraining....also can alter your PE.
There are, as you know, a few different "accepted" methods of determining your LT in the field. Joe Friel and Chris Carmichael both offer widely accepted tests. Although they differ in methodology, I've found the results to be the same, regardless. During the 30 minute test mentioned earlier, I would eliminate a prior warm up. Since you're only capturing the last 20 minutes of the test, the first ten should suffice as a "warm up" period.
And finally, regarding HR during competition and competitive TT's: HR during competition can be deceptive. Due to the extra "stress hormones" that are released, your body can maintain HR's a little differently than during training. The important thing to remember, is simply fuel storage capacities and limits.
Feel free to read this:
http://www.scribd.com/doc/25586695/B...lasmy01r2161mk. It's VERY basic, but may help to give a little more insight.