Better or worse than average?

It's probably easiest to focus on people who don't seem to improve as much as others.

In my experience, there are three reasons why this happens:

1. Inconsistent training. They train 14 hours one week, 3 hours the next, and not at all on the following week, before trying another 14 hour week on week 4. This isn't necessarily their fault: perhaps they are not in a position to make training as much of a priority as others. As well as never getting fit, they're always getting injured ..... as the old saying goes, they live in three states: injured, about to get injured and coming back from injury.
2. Fundamentally they're not designed to be endurance athletes .... they might be great football players or powerlifters and there's no reason they can't participate in and enjoy triathlon but, they're never going to be as good as those of us who were designed for endurance sports.
3. Not enough variety and progression in their training. They do all of their training at more or less the same pace and their volume and balance remains the same from year to year ... I'll talk about this some more in the next post.

The exact opposite reasons are the reasons that other people make faster than average progress i.e.

1. They are very consistent and maintain workload over time i.e. their week to week volume rarely changes much and therefore they rarely get injured.
2. They are naturally talented in at least one of the disciplines
3. They push themselves in training .... even if their training volume doesn't increase from year to year, the pace that they train at does.