Hash different varieties, different sieves (confused…)?

In the classic production of dry sift, we are simply talking about dry sieving…

Now there are different sieves…

70 micrometers

90 micrometers

100 micrometers

120 micrometers

160 micrometers

According to my mathematical understanding, the hash filtered at 70 micrometers should be finer and therefore better than the hash filtered at 120 micrometers.

All of my secondary school dealer colleagues are convinced of the exact opposite. They don't talk about micrometers, but rather U (it's the same thing), but for example they tell me that Drysift starts at 75U and then goes up to 160U, so 160U would be the best… huh?

I'm only finding more confusing posts about this online. Can someone please clarify it for me?