The explanation given above (by Gandalf3) is accurate, but a little hard to understand for those new to Blender. It may help to clarify how this problem comes about and how to fix it:
When creating bones, Blender will automatically generate a vertex group with the same name; which links the vector weights to the relevant bones. However, an associated vertex group may not be generated if you import an armature, and sometimes when you duplicate / copy-paste / rename bones (or delete vertex groups).
So, if you've got a 'head' bone that's connected to a 'neck' bone, but only have a vertex group with the name 'head', any weight painting done over the neck area will result in a 100% weight. This happens as Gandalf3 explained... if there are no weights (from other bones) controlling those vertices, Blender calculates all weight values applied to the area as 'the only thing that needs to be considered' = 100% influence.
To fix the issue, you either need to add bones, or determine which bones don't have vertex groups and create them (with the same name as the bones - and assign vertices to them).