windows New PC Windows automatically activated?

[Rekoran]

Hello guys, as the title says..

Im workin in an PC - Store where i have to build PCs. 
In the most cases, when the PC is finished building and i install windows, it isnt acitvated. This is what i expect when building a complet new pc with new parts. 
But on some PCs, when installing windows, its activated automatically. I dont even have to open the License to look at the key, because its activated. Without doing anything. 

I cant find anything to this in the internet. So im asking here, 

why is windows activated on some pcs? It happends on 1/10 pcs.

Thanks in Advance

[MikeHawkSmol]

Did you log into your windows account? I bought one win10 license, and have 5 PC's running, logged into my Microsoft account, and all have windows activated through the original license.

[Rekoran]

For sure not, because i build the PCs for Customers. 
And when installing windows, i usually do it without Internet Connection so i dont have to create a microsoft account.