People TEND to use their highest parse they've seen when making a benchmark for DPS. On top of that Black Mages are very RNG based. Sometimes you'll get 2 firestarters and a thundercloud in your first Astral Fire burn then never see a proc for minutes. Due to this, sometimes when someone says "over 400" what they mean is "I'm a decent black mage who got a million procs, then we killed the boss, and I saw how much I did" (especially on T8).

That said, Black Mages having "not much variation" is indeed much more myth than fact. But it comes not from proper rotations as most Black Mages I see are at least competent with their rotation. Where the lines start being drawn is when movement becomes an issue. Fights like the ex primals and T6 have a lot of movement and a talented black mage who knows how to manage his procs and aetherial manipulation to move about the battlefield and use their Manawall/Ward to ignore mechanics that would cause them to move will easily do 20-30% more DPS than a Black Mage who follows the A-B-C rules and nothing more.

tl;dr take everything blms say about their numbers with a grain of salt. We have a heavy rng component of our rotation, variation is going to happen and people like to use the best of themselves as marks.