Annotating RRBS methylation data with Illumina cg number

Question

[SimH]

I have RRBS data (Reduced representation bisulfite sequencing) that gives me the percentage of methylated "C" at a given position in the genome. I would like to compare this data with other datasets I have already established previously using MethylationArray data (using the Illumina 450K or Illumina EPIC). I therefore tried to add the "cg" number where possible but I only got ~ 80,000 sites added (of ~850,000). I did this using dplyr in R with joins. Changing the annotation +/- 3 did not change much. Both, RRBS as well as the annotation data is aligned with GRCh37, so this should be okay. Does anybody has a suggestion of has done this already? Thank you very much for your help!