In a couple of cases recently, I have seen the app (I think) indicate J0 Y0, but the standby tab show Y as "available." It may be that one or two seats pops open for a moment before someone grabs them, and the app isn't updating in real time.

Alternatively, it may be that United is capping inventory below the total number of seats on the plane. Could it be a weight issue, or that UA intends to fill the hold with cargo such that they can't take a full passenger load? Or maybe an inoperable seat?