Hi Ivo,
That is interesting to note, here are the possible reasons for these differences
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If you have not noted already, since January 2022 ESA updated the Sentinel 2 processing baseline, as reported in this post, Sentinel Hub has since implemented parameters to harmonize the values, before and after the baseline change, see docs . I have no information whether this has been implemented yet in GEE. This is worth checking , seeing that significant differences appear in the months after January 2022
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Processing options such as cloud filtering maybe quite different, it seems there were abit more cloud free dates selected in EO browser as seen in the chart. Perhaps you could compare the cloud filtering options you did use for GEE , even better could you share the GEE script ? that might make the comparison easier
Best regards