Why is Amazon buying from us?

We have been selling FBM on Amazon for about 14 months. In that time, we have received several orders from Amazon that shipped to whichever Amazon Warehouse was specified on the order. Any thoughts why would Amazon be ordering from us as a customer? Has this happened to any other FBM sellers? Are they just checking for authentic goods?

Note: We are an authorized distributor of industrial products, so the items that we sell are US brands such as 3M, Black & Decker, Honeywell, etc. Amazon is often also a seller on these same listings