While I get what you're saying, but I'd assert that there should be a dialogue between the narrative legends of a monster and how that plays out with its abilities. 
For example, if you ask "what provokes legends about this dragon?" and then look to its Legendary Actions...
"It slapped three knights with its tail!" Um, OK, that's cool, but if I'm a bard I'm probably not going to make that the highlight of the monster when I tell a tale about it at the tavern.
"It spotted the party burglar hiding in the shadows." Ok, and so could a lot of monsters.
"It beat its wings, scattering a small force of 16 men and then took off into the sky...but all the knocked down men got to stab at it with their spears before it took flight." Wow, so that's pretty cool! I could probably tell a legend about that, if I edited out that last bit.
As an aside, I tracked down the origins of the Legendary Creature in D&D because I was curious if the current incarnation had left anything out. There were 2 sources: The AD&D High-Level Campaigns book (with a rather mediocre treatment focused on selecting traits/abilities from an uninspired list) & the AD&D Planescape Monstrous Compendium Appendix 2 (which is simpler but much more evocative in terms of story, and emphasizes extraordinary defenses straight out of Greek myth). 
http://www.enworld.org/forum/showth...-Planescape-Origin-of-Legendary-Solo-Monsters