Not terribly surprising to me. It was caught from the largest reservoir in the state that is in the southern half (longer growing season), has high productivity (fertile), only moderate largemouth bass population, plenty of pelagic forage (shad), minimal submerged weed growth and a minimal abundance of smallies (minimal competition) that usually only show themselves for a month or so in spring. You typically see a smallie or two every year caught from there that go 3-5 pounds. It also has a very small spotted bass population. I could see that state record being broken from there, also, in the future. It is now nearly 60 years old, so there has been plenty of time to wash away the dirt that covered all the limestone and clay that primarily makes up the geography of the area, creating small portions of the lower lake that are more suitable for smallmouth.
There are two other waters that could eventually produce another smallie of that size down the road - Patoka Res. which is the second largest reservoir in the state, same basic characteristics, but a slightly more abundant smallie population and a little better overall habitat (cleaner with more rock and clay). There is also the IN portion of Lake Michigan which gives up some nice smallmouth every year, but doesn't get near the pressure of most lakes or other sections of the Great Lakes due to its location.