Because they are showing last RECORDED taxes by owner at that point in time. My home showed $890 after 9 years of previous owner and it was $1600 on my first bill. Depends on exemptions of prev owner and how long they have been there. Florida has a law which (for full time owners) holds the price based on some kind of index for change called SOH (save our homes). If you move in you start paying "full list" (so to speak) based on current valuation despite what previous owner paid. Only in new(er) locations will they be closer. Best place to look is on your county tax collector site. You can see every detail history of every home in the county.
This is for Sumter county:
Sumter County Property Appraiser Put in 1 address then page through every other home on your street.