For board exams and as a rule of thumb we say bone is 1000 HU; however that is not the complete truth. While a compact bone is typically high in HU, making a measurement of cancellous bone is much more difficult. It inherently incorporates other types of low HU tissue that is averaged into the measurement and reduces the average HU value. This measurement can be as low as 200 HU depending on the placement and size of the ROI (see image below)
As an analogy think about how your dish sponge might be yellow in color, but it is inherently filled with colorless air and so any measurement of its “yellowness” will be brought down by the colorless air inside of it.