The con level ranges as I wrote them based on the OP question is correct.
I just dinged a L55 beastlord, who was fighting this mob. Notice there level range. http://eqbeastiary.allakhazam.com/search.shtml?id=24602
They were all DB, guess what some of them did when got to L56? Yes, some went LB. Same thing with a L60 bard, they were all LB, ding L61, and now some of them are green. For the beastlord some are now -6 levels, and for the bard some are now -11 levels. Hence the con change for about 1/3 of them.
Now on the matter of under L20 game the cons are not all there.
L1-9 there is no green/LB con- Mob DB, ding they go grey.
L10-16/19 range there is no LB con. One mintue there DB, ding the next they are green cons. You have to get beyond L19 to start seeing LB con mobs. Then the system works from there. And so far it works that way.
Again beyond L69 I don't know if SOE expanded the range, or not. I will find that out soon enough.