It's an interesting question but, I couldn't hazzard a guess.
Also, there can be accidental overdoses- in the most likely case of Heath Ledger, Elvis Presley and many more I suspect. Then, there are others where it may been a suicide attempt- Kenneth Williams for example. That may well have been accidental but then, he did seem depressed too so- it would fit.
If you're wondering about the real intention behind an overdose, that's very hard to judge. Even harder to quantify. I have known someone who overdosed. They described it more as being in such a desperate state, they weren't exactly thinking logically. I think they did want to suicide although, common sense would suggest to me that they wouldn't have achieved it via what they had. They were fairly young at the time though so- maybe they didn't realise this.
Not too sure what you're pointing at here though- there seem three main types really- accidental overdose, deliberate overdose as a suicidal gesture (that God awful phrase: 'A cry for help,) or, a genuine suicide attempt. It can be hard to even assess whether it was accidental or deliberate sonetimes- if there was no note left. Especially if the person is a habitual drug user- prescribed or recreational. Let alone what was really going on inside their heads before they took them.