Why The Discrepancy?

In a post a few days ago, Bill complained about my comments regarding arrest for a lorry driver in a pedestrian collision, saying that:

Regardless of fault he did kill someone. The whole point of the arrest is to establish if he was grossly negligent. That can't be decided there and then.

And frequent commenter Blueknight noted:

The Police have to abide by ACPO's Road Death Manual. ..if an early assessment indicates that a criminal offence may have been committed, arresting suspect(s) who have been identified and are present at the scene of a fatal collision must be a priority, for example, in cases where impairment through drink or drugs is suspected. In such circumstances, the escape of any potential suspect(s) and the intentional or accidental destruction of material (e.g., tachograph charts, documents, and reconnection of speed limiter) must be managed. The isolation of any suspect(s) and/or vehicle(s) is also important ..

Well, this incident occurred recently:

A teenage boy has died after being hit by a car on one of London’s busiest roads. The 14-year-old died yesterday following the accident on the westbound North Circular Road near Brent Cross on Tuesday afternoon.

What is noticeable about this one is as follows:

The driver of the Mercedes estate car involved stopped at the scene and was not arrested.

Now…why not? How come the police were concerned that a criminal offence 'may have been committed' in the first incident, and yet not in the second?