I actually ran into something along these lines recently looknig for tickets for a trip around Christmas. Google flights would price out ~$500 a person, but when I clicked to buy on Delta it would only sell me an "A" fare for one of the flights, bumping it up by several hundred. So I called in and the CSR got the same price that Delta did. I figured maybe they'd just sold the last ticket in the cheaper fare class or something. But it continued for weeks. I'd get one price on Google flights and then a significantly higher one when I clicked through to Delta. I even tried just buying one ticket to see if maybe that was the issue, but no luck.

Basically Delta was telling ITA that there were tickets available in coach, but refusing to sell them to me.