There are many reasons and a fan of Genealogy Tip of the Day on Facebook gave an excellent example: county boundary changes.
These three pages (title page and pages relevant to my searching) actually are not from compiled state statutes, but rather are from the "Acts Passed at the First Session" of the 25th General Assembly of Kentucky in December of 1816. Session acts can be just as helpful as the compiled state statutes. The images that follow describe the change in the borders of Bourbon County, Kentucky--which explains why my ancestor's deeds of acquisition were recorded in one county while the deeds where it was sold were recorded in another.
- slip laws-published immediately after passage
- session laws-all the slips passed during a specific session, usually published after the session is over and in chronological order
- compiled statutes-laws in effect in a jurisdiction at a specific point in time, usually arranged by topic and reflecting the most recent legislation passed
I blogged about the specific change here--with a map to show the line change. The "straight line" that originally formed the northeast border of Bourbon County was changed out for geographic features.