PC p values in JackStrawPlot in Seurat are not in order

Hi,

I got the Seurat object, then I did:

pbmc <- JackStraw(pbmc, num.replicate = 100)
pbmc <- ScoreJackStraw(pbmc, dims = 1:20)

And when I did:

JackStrawPlot(pbmc, dims = 1:15)

The plot looks like this:

As you can see, the p values are not in order (from small to big) like the tutorial in Seurat tutorial. I wonder why this could happen and how to solve this?

Thanks very much!
Leran