False imprisonment is usually a lesser included offense for kidnapping.
http://en.wikipedia.org/wiki/Lesser_included_offense
Basically, a lesser included offense is conduct that is considered less serious under the law, but its elements are also included in the elements of a more serious offense. So, false imprisonment is a lesser included offense for kidnapping; larceny is a lesser included offense for robbery; assault is a lesser included offense of attempted murder; etc.
For the State to charge Kibby with both charges, there has to be separate conduct that fits the elements of both crimes. So in Jaycee Dugard's case, there were instances where she was kidnapped, and instances where the facts fit the crime of false imprisonment but lacked the facts to make it kidnapping. It's a matter of elements - each charge has a list of elements that have to be met to charge someone with a crime.
So, for false imprisonment, you have to have:
1) a person knowingly
2) confine another person (the victim) unlawfully
3) in a way which substantially interferes with the victim's physical movement
But for kidnapping, you have to have:
1) a person knowingly
2) confines another person (the victim)
3) with purpose to:
- hold the victim for ransom or as a hostage; or
- avoid apprehension by LE; or
- terrorize the victim or some other person; or
- commit an offense against the victim.
So, unless there are two separate instances, where Kibby met the facts of false imprisonment, without the added elements of kidnapping, there wouldn't be any false imprisonment charges. Does that make sense?
[NOTE: New Hampshire also has a subsection of the kidnapping statute which states "A person is guilty of kidnapping if the person knowingly takes, entices away, detains, or conceals any child under the age of 18 and unrelated to the person by consanguinity, or causes such child to be taken, enticed away, detained, or concealed, with the intent to detain or conceal such child from a parent, guardian, or other person having lawful physical custody of such child. This paragraph shall not apply to law enforcement personnel or department of health and human services personnel engaged in the conduct of their lawful duties."]