I've never had a spec that would thoroughly describe how to handle every case. The ability to understand and interpolate vague business requirements is expected to scale with your level; if you take a "not my job" approach to such decision-making you are perpetually entry-level.
I've often seen the exact opposite of this: entry-level devs being all too happy to 'fill in the gaps' in a spec based on their own (very limited) understanding of the problem domain (not even being fully aware of where the spec ends and their interpretation begins), and more senior people recognizing when they don't actually know the answer and realizing that it's important enough to clarify with the domain expert. Basically Dunning-Kruger effect.
