I don’t think the ethnicity is a factor-- but certainly essays & LOR’s might be. Perhaps along with the great test scores and terrific grades there may be other issues that teachers find concerning. (Some that come to mind, not necessarily for this particular example, could be, a student who is perceived as being focused on grades but otherwise incurious; a student who is perceived as self-aggrandizing and arrogant; a student who is believed to be a cheater; a disciplinary issue that was handled quietly at the school but needs do be disclosed to the colleges; or some personality problem completely unrelated to the academic stats). Many LOR’s damn with faint praise, and in some cases teachers or g.c.'s may be more direct in letting colleges know of a potential problem. A student who has top grades and test scores who gets rejected from every top 10 school he applies to has some other issue. (Heck, who knows, may just be that a kid is lying to his friends about his grades and test scores – or maybe the kid plagiarizes his college essays)
And yes, the essay itself could be a factor, if it is offputting in some way. I don’t think that the vast majority of essays make a difference, except at the extremes – the ones that are unusually strong or compelling can tip the balance toward admission — and the ones that are extremely poorly written, or offensive in some way, or raise red flags – can definitely lead to rejection.