If you have ever been frustrated about how much your grocery bill fluctuates each week, it’s not in your head: Your eggs might cost more than your neighbors’ if you shop for them through Instacart, according to a new report.

In a joint investigation, nonprofits Consumer Reports and More Perfect Union and progressive policy group Groundwork Collaborative teamed up with researchers to recruit 437 volunteer secret shoppers in-person and online across multiple U.S. cities. The shoppers placed an order for the exact same set of basket items at the same time and at identical stores.

The report found that Instacart shoppers could receive up to five radically different prices for the same everyday household items like Wheat Thins crackers, dairy or cereal. A dozen Lucerne eggs could either be sold for $3.99, $4.28, $4.59, $4.69 or $4.79 on Instacart at a Safeway store in Washington, D.C., for example, while a box of Good & Gather Farfalle Pasta at a North Canton, Ohio, Target store got priced at $1.19, $1.39 or $1.43.

These price differences could add up to a lot over a year of groceries. Some shoppers in the study got prices that were up to 23% higher than prices available to other shoppers for the exact same items. In total, these price swings could add up to $1,200 per year, the report found, using Instacart’s own analysis of what an average U.S. household of four spends on groceries.