UV Randomiser Node Tree

I've been looking to find a way to easily randomise UV coordinates for scratches on metalic objects. My solution has been to use a small node tree to help drive this. This works mostly, but there are some things which I don't quite understand. The Seed value I am using seems to have ranges where the randomising breaks. And I don't understand why.

Here's the randomising bit.

And my shading network.

And the results. This is only working because I am using a specific seed value. I think in this case it's a value of 0.900.

Can I post a link to share a .blend file here?

I would be very interested to be pointed in the right direction. I suspect that I am confused by how Blender handles data and what happens when one makes copies. Pointers to reference materials would be appreciated. Thanks!