I shop every year too. Every insurer has their secret sauce formula, and just because you are in insurer A is the best deal now doesn't necessarily mean it will be the best in the future. Especially early in your driving career, the competition is fierce and quotes vary wildly.
Since the formulas are secret and you can't run them all at home, the only way is to get quotes regularly.
As your driver history lengthens and your vehicles age (or are replaced), you may fall in or out of different insurers' sweet spots in their respective pricing functions.
edit: just for fun, here is the last history of my auto prices, for 6 months of premiums. My car has always had liability only, wife's always had comprehensive (with the highest deductibles)
2012: $192 (Allstate), $450 (Farm Bureau)
2013-2014: (moved to another town 10 miles away) $240 (Allstate) $400 (State Farm)
2015: $350 for both (!) after getting married (Geico)
2016: $400 for both, still Geico. Everyone raised their rates due to some redistricting, or so they said. Still cheaper than everyone else.