For a word like 'safe', or at least in CS, I would assume that the 'safe' one actually is 'safest'; that 'safer' is ehh it's not safe but it's an improvement on the unsafe one. It's safer.
One example to help think about this. Say you have 3 friends. Your friend Bob has a net worth of $1 - he is the least rich. Your friend Alex has a net worth $10 - he is richer. Another friend Ben has a net worth of $100 - he is the richest. Richest here is comparative against all 3 of them, but none of them are actually rich. Bill Gates is rich. Bezos is rich. Musk is rich. Someone with a net worth of $100 isn't.
You can still have comparisons between the rich too, so Bezos is richer than Gates and he's also the richest if you're just considering the pair. But add Musk to the mix, and he's no longer the richest.
I guess that last example looks like you have two attributes - rich as some objective "has a lot of money" and comparatively rich (richer, richest). For safe, it's kind of similar, except that as soon as you are saying one thing is safer than the other, then you are implicitly acknowledging that there are areas where the thing isn't safe, and if you're admitting that you can't also call it safe without contradicting yourself.
A better example is "pure water". By it's definition, that's just H2O molecules floating around with nothing else.
If you add a single grain of salt to a glass of that water, it's no longer pure. Drinking it you probably wouldn't notice, and some people might colloquially call it "pure", but we know it isn't because we added some salt to it.
If you add a teaspoon of salt to to a different glass of pure water, it's also no longer pure, and now most people would probably notice the salt and recognise it's not pure.
If you add a tablespoon of salt to to a different glass of pure water, it's definitely not pure and you probably wouldn't want to drink it either.
You could say the teaspoon of salt glass is purer than the tablespoon of salt glass, the grain of salt glass is purer than both of them and so the purest of the three. And yet, we know that it isn't pure water, because we added something else to it.
So pure > purest > purer > less pure. Also note that I was required to use "less pure" for the last one, because all of them except pure are "impure" or "not pure", even though were what I originally thought of writing.
It's a bit ambiguous and depends on context, which is why I said 'at least in CS', since for whatever the particular topic is 'safe' and 'unsafe' is likely to have a fairly strict meaning.
In general you're right. For safety it's just that 'safest' implies some sort of practicality: the best - most safe - from a set of options. But the safest option isn't necessarily strictly safe.
(Say your dog's stuck on a roof on a windy day, you decide the safest option is scaffolding (safer than a ladder or free climbing), but it's not safe, you just insist on rescuing your dog.)
