Why might variable expenses change a great deal at different times of year?

The variable expenses could change a great deal because the market value of each variable expense could fluctuate throughout the years.

For example if a company produce Ketchups, it need Tomatoes as one of it materials, which mean the price of tomatoes is one of their variable expenses.

The price of tomatoes would be a lot cheaper during growing season compared to the winter, where it would be harder to cultivate the tomatoes