From the wiki on the slave hunters page.
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Incapacitation chance
- Base: 5%
- Fists: +80%
- Whipping cane: +85%
- Combat staff: +70%
- Sledgehammer: +35%
- Knife/sword: +15%
- Great sword: +15%
- Firearms: -3%
- Heavy firearms: -4%
So, if we take this at face value, using a whipping cane gives you a 90% chance of incapacitating an enemy taken to zero health by it. (Barring the immunity conferred to some enemies like bounty targets)
So this would explain why some people are getting enemies killed on hit, there is a 10% chance of it killing even when using the whipping cane.
So why the disparity in experience then?
Well if we remember the Manhunter trait, which confers another +15% chance to incapacitate then this takes the final total up to 105% chance to incapacitate, so it may well be that the people who are reporting certain captures opted to take the trait while those who occasionally have a capturable character die on them did not, falling victim to the 10% chance of death they would be subject to without the trait.
Obviously there may be something I am missing here, but I think it explains things?