Mistakes were made

I've been trying to crank out some pieces for a crafts fair I'll be participating in on March 20. It's for the Interfaith Families Project Purim Festival. I was an active member of IFFP for many years--it's a wonderful group for Christian-Jewish intermarried families. In addition to the crafts fair part--which I gather is pretty small (they tried this for the first time last year, and I think it was limited to a couple of teens selling jewelry)--there is a silent auction. I spent today making a necklace and earrings for the silent auction--I'll post a pic here when I can take some better shots.

The photo above is for a piece that's still in progress. I wanted to make a Russian spiral rope necklace. Instead of making one long rope, however, I envisioned making it in three segments, with some large beads interspersed in the breaks. I did one 7-inch segment, the one on the left, on Monday or Tuesday. The following evening, I started segment two, but for some reason it came out a lot skinner. I took it apart several times, but it still stayed skinny. I assumed that I had messed up on the set-up row. I finally gave up, and the following evening I started a new segment, taking extra care with the setup. It, too, came out skinny. Then I did two more, both skinny. I can't figure out why the first one is so much fatter than the others. I can only assume that I somehow added a bead after the set-up row, and I can't replicate my mistake. Either that, or it's simply a matter of tension. I know that with many tubular stitches, the first inch or so is always loose, but I can't account for this. I will save the fat one for another project and use the three skinny ones for my current design.