[BioC] Infinium 450k beta value distribution

[maria giulia bacalini]

Hello everybody,

I'm very new of R and Infinium methylation analyses.
I'm trying to analyze beta-values from an experiment performed by a collegue on Infinium 450k platform.
When I plot the distribution of the beta-values, I achieve a distribution with 3 peaks: 1 around 100% methylated (expected), and 2 around respectively 0 and 10% methylation, like in the figure attached. I expected only a peak in this region, i.e. a bimodal distribution. Do you think this could be a normalization or techinical bias? Has anyone ever observed something similar? 

Thank you in advance,
MGiulia