This could be for a couple of reasons. Although I am using the image for example, I know you mean in general so I guess I also mean my answer can be taken for in general.
1. The player is using the headset to farm pieces right now, so that is probably not the final gear.
2. Another thing could be that they were preparing it for a different set. I've seen some players have a strategy of separating the 3 gear between two pieces. So for example maybe they have a shirt that is "Special power up" with two slots on that and one with "Primary ink saver".
The reasoning is very simple, you get the same result if you put them on as if you had them pure but you need fewer ability chunks if you accommodate everything between the two.
To make two pure clothes you spend something like 10xa 20xa 30xa & 10xb, 20xb 30xb, = 60xa +60xb
if you use two gears you spend 10xa 20xa 10xb & 10xb, 20xb, 10xa = 30xa +30xb
So if they are sure you will use both pieces of clothes together, so maybe that's why they have that particular one like that.
Also, not everyone is into pure sets. I know I am not. I rather the Primary slot to be something I can't get for the smaller chunks like Tenacity. I guess different people just play it differently.