How subjective is the recruitment process?

Like many on here, I'm looking for a new role at a higher grade in the civil service - more specifically, going from EO to HEO.

And, whilst I've been in the CS for a while now, like many others here, I have noticed vast inconsistencies in the recruitment process. So, what exactly are hiring managers looking for?

If I were to read the job spec, it would suggest that they need people with X, Y, and Z skills and/or abilities. I can tell them that I have X, Y and Z abilities. By contrast, if I look at the behaviours, they want people to demonstrate A, B and C.

So, which is the more pressing priority? Of course, one should attempt to forge behaviours that apply or relate to the job at hand. But, is a hiring manager going to penalise you for not hitting each and every point in a behaviour? Or do they have the ability to pick and choose which elements are important? Is the broader example more important over the nitty gritty?

I'm not so naïve as to suggest that the hiring process is perfectly objective - but just how subjective is it and how much can hiring managers strictly deviate away from behaviours? I'd just like to reduce to opacity to some degree.