Well yes, but the reason people don't understand the seriousness is generally thought to be that when in groups people will see the inaction of the group and conclude that if it was something serious, someone else would have responded and/or assume they have intervened (e.g. by calling an ambulance).

In groups people will pretty consistently take longer and be less likely to even notice what is going on, and then take longer and be less likely to respond, even when the situations are otherwise identical, because people are inhibited by social conventions and "social proof".

There are factors that may reduce that difference. E.g. the more clearly it is an emergency. But even situations where people are very obviously in distress can result in inaction - the Wikipedia page has several examples.

Cialdini (in Influence) notes that even specific cries for help in obvious emergencies are often ignored by groups. He himself was actually in a car crash, and realised people were just driving by and ignoring them, until he realised and made use of his own advice: Make yourself noticed, be specific about what help you need, and be specific about who you want to provide it. That is, screaming "help" gets you ignored. Screaming "you in the blue jacket, call an ambulance" gets people to respond. Once you first get a response, you get more response - the bystander effect works to your benefit: people see other people acting, and suddenly everyone accepts it's an emergency and helps.

> If I knew the person was requesting help – even if I don't know WHY they are requesting help – I would respond.

That may be true for you, especially if you are alone. But the moment group dynamics are triggered, the odds starts dropping that you will intervene with each extra person involved up until some fairly low limit (at which point the chance of intervention is quite small).