Florida
Best Decade: 1970s (+4.20 Seats)
Reason: IDK
Worst Decade: 1910s (+0.43 Seats)
Reason: Maybe the state was seen as a backwater back then
Not the only reason, but the answer to both is air conditioning. Florida was a slow growing backwater before the 60's or so, because it was too damn hot to live in. In the 50's almost every other southern state had more people then Florida did, and when air conditioning became a big thing in the late 60's/early 70's while all southern states grew by a lot, Florida was even cheaper and prettier then the other places in the south. Also, the 70s were a big time for all the southern states because thats around the time northern businesses decided to flee en masse south for cheaper labor costs.