I am basing my qualifications on what the game offers, what makes sense.
Normal mode drops 252 ilvl gear (and a few 259). LFR Drops 239 (and a few 246) If you are purely a raider, it's logical to gear through raids only, and playing M+ should never ever be mandatory for pure raiders. That means, that the highest possible ilvl you can get from doing only LFR in theory is about 240. And if you also do Zereth Mortis, which shouldn't be mandatory for people who only want to raid, you can get about 246ilvl with the catchup gear then.
That means that asking for more than 246 for Normal Sepulcher is absolutely ludichris and madness. There is no plausible way to get higher than that for a raider. And once you reach 252-253ish, meaning full Normal raid, where Zereth Mortis gear can't even help you, there is no plausible way to get higher gear before Heroic, hence 252-253 SHOULD be the requirement to join Heroic, anything above that, again, is madness to ask for Heroic.
Yet, just now, the LOWEST I could find, after searching for almost 2 hours, is 260 requirement for heroic and many ask for 270+. That is COMPLETE insanity. HOW IN EVERYTHING THAT IS HOLY am I suppose to get 260ilvl without doing Heroic? The community is crazy.