Does anyone work with the Ribogreen assay to measure the siRNA concentration?

I have tried it several times but then the standard curve although it's linear but it's very different from the one on the manufacture protocol.

Moreover, when I start with known concentration of siRNA and try to confirm it through the Ribogreen, I found it higher than the calculated by about 20%, is that correct and which one should I use, the calculated or the experimental?

Thanks