Why are items priced at $0.00?

I deal with this multiple times a day as a cashier. Scan a code, then it asks “how much is this item?” Zebra shows it’s $0.00. If there’s no tag either then I either price match it or if all else fails just give the guest a lowball guess that they’ll be happy with. But what’s really going on in the background? Why is it at 0.00 to begin with? Or is it just one of life’s little mysteries?