The default Blend Mode for Layer Groups is called “Pass Through“. It allows any Adjustment layers inside the group to affect all the layers below it, including layers not inside the group. In other words, the group is simply used for organizational purposes.
If you change the group’s Blend Mode to “Normal“, none of the Adjustment layers inside of the group will apply to layers outside the group. The group is no longer used for just organization, and it is now looking at the content inside of it as a “flattened layer.”
Make sure to check out my article, Photoshop Blending Modes Explained, where I discuss everything you want to know about what Blending Modes are and how they work.