I have a question about fusible grid!

When I was working on my first watercolor quilt I used fusible grid, which was very helpful and convenient!

I don't remember exactly what brand it was and didn't save any labels, but I remember I used 2" squares of fabrics for it. You can find more about this quilt here.
Since then I still had some leftover of this grid and I started working on my new watercolor project. When I used it all, I bought another couple yards of Pellon Fusible Nonwoven 1" Grid.

And when I fused the rest of my 3" squares I was surprised and confused: they didn't match with the first part of the project!!! I thought all of them should be 1" grid, but the difference was about 1/32" per square, so the first part now is smaller about 1 inch for every 32 squares of the grid!!! I was very disappointed and again I said to myself: when you prepare for the project, you need to bye all ingredients from the same bolt!! I've alredy had the same kind of problem when I was lack of blue Robert Kauffman "Fusion" collection fabric. When I found and bought what I needed online, the color was slightly different! Nobody but me could notice it in my project, but I was the personal witness! And I didn't realy like it!
Well, in this case I found the solution. Since all fused squares are 3", I layed my 2 1/2" long ruler in the middle of 3" squares and draw line on both sides for the seam.

Seam allowance will be sharp 1/4" on the grid I recently bought, and slightly shorter on the old one, about 1/32" form both sides.
Now here's my question: why the two grids were so different? I was pretty much sure the previous one was also 1" square, but eveidently not!!! Does it depend on brand? On the width of lines? Or it shrunk over the time? Does anybody have a clue?

p.s. well, looking through my own blog I found one more watercolor quilt, which was one of my firstest!

You can find it here. Back then I even didn't know about fusible grid and I made it old-fashioned way - chain-piecing. But to be more precise - this one was my very first watercolor quilt!

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