No one who does the assessments think they're fool proof, but trying to determine age (to such a specific range) by physical characteristics is about as reliable as trying to predict criminality based on someone's skull shape. With the same undercurrents of non white races being more likely to be discriminated against. So unless you have a better option, it's what they've got.
The guidance is that if it's borderline, to assess as younger. They can only assess as over 18 if it's very clear that the young person is well over 18. I know some people won't like that, but the alternative is risking genuine children being put in adult services and not receiving the care and protection they deserve, so they have to err on the side of caution.