My understanding is that this is partially due to different pasteurization processes. Organic milk is heated at a higher temperature and for less time, than the pasteurization process used for non-organic milk. This carmelizes the sugars in the organic milk, making it sweeter. The process is also the reason why organic milk lasts longer on the store shelf.

See this article: https://www.scientificamerican.com/article/experts-organic-m...