Inventory Cost keeps increasing after each sale

Hi I have sold 5 items of the same inventory bought at the same cost. My cost price is $3.50.
The first item sold was costed at $4.09
The 2nd at $4.63
The 3rd at $5.52.

I have not other expenses added.

Kindly assist if you have an idea why this is happening.

What is happening though? When you sell 1 item where cost was $3.50, regardless how much you sold the item for, Manager will automatically credit Inventory on hand and debit Inventory - cost expense account by $3.50. Is this what you refer to?

I think I made a mistake with my double entry .

Kindly advice on the account to DR and CR when inventory is purchased and when it is sold. So I can correct this issue.

The item sold cost $3.50 and was sold for $10.

When inventory is purchased, categorize the purchase to Inventory on hand account, then select inventory item.

When inventory is sold, categorize the sale to Inventory - sales account, then select inventory item.