Sales boost switching to free shipping, or fluke?

Small single-product FBM seller here since July ’24. I always charged for shipping. My sales were always decent ($2500-$3000/mo). Well I decided to switch to free shipping and increase the item cost to the average shipping rate I was paying for the previous 6 months plus $1.

Well my sales went up, a LOT. I’m only 3 days in this month and I’m already at 30% of my average monthly revenue. I ran out of stock (I make my own product).

I don’t do PPC or social media. So does Amazon show your listing to more customers when you do free shipping, do customers just prefer to not pay shipping as a separate line item, or is this just a fluke?

Edit: my items are for business customers, typically solopreneurs.