[Navigation Timing] [Resource Timing] Why do some phases not have an explicit end time attribute?

OK,

I need a history lesson - I've ad a hunt through the archives but can't
fnid the answer...

Why do some phases of the processing model have explicit end times e.g.
redirectEnd, and others have inferred end times e.g. AppCache, TCP, SSL
etc.?

Can anyone shed some light on it for me?

Thanks

Andy