A carton of cereal costs roughly the same all year, while the price of lettuce or berries can double and halve within a season. The difference comes from the fact that produce cannot be manufactured to demand.
Supply arrives when the crop is ready
A packaged goods factory raises output when demand rises. A field produces what it produces, on the schedule the weather sets, and cannot be asked for more.
Harvest windows for many crops are narrow, sometimes a matter of days for a given block, and everything picked has to move immediately.
When several growing areas peak together the market floods and prices collapse. When a window is missed the shortfall cannot be made up later in the season.
Weather transmits straight into price
Heat, frost, drought and rain at the wrong moment reduce yield or quality, and quality losses matter as much as volume because unsaleable fruit still costs the same to grow.
The effect reaches shelves quickly, because most fresh produce is sold within days or weeks of picking rather than out of long-term storage.
That is why an event in one growing region can raise the price of a single item nationally while everything around it stays flat.
Production moves between regions during the year
Most fresh categories are supplied by a rotation of growing areas, moving north and south with the seasons and crossing borders for part of the year.
Each transition between regions introduces a gap, because the outgoing area is finishing while the incoming one has not reached full volume.
Freight distance changes at the same time, so the same item can carry very different transport costs in different months.
Storage capability separates the stable items
Potatoes, onions, apples and squash keep for months in controlled storage, which lets supply be released gradually and smooths their prices across the year.
Berries, leafy greens and stone fruit have no such buffer. They must be sold quickly, so price does all the work of clearing the market.
The most volatile items on a produce counter are almost always the ones with the shortest life after picking.
Retailers absorb some of the movement
Stores do not pass through every swing. Staple items are often held at a steady retail price while the store absorbs the cost change, because visible volatility on a known item damages the price impression of the whole shop.
Loss on those items is recovered elsewhere in the department, and the smoothing is funded by margin on less closely watched lines.
Which means the prices a shopper sees are already damped, and the underlying market moves considerably more than the shelf does.