Seems like United is discouraging same day quick turn (domestic) with its fare mapping.

Just doing some "research" of random routes - e.g. EWR<->LAX. Same day return within 4 hours after outbound scheduled arrival, with travel date 2 months from today (to minimize any AP issues) - totally available G or K inventory on both ends would auto remap to W, and fare would more than double.

The fix.would be to extend more than 4 hours between flights, or book two one-ways.

Very interesting...don't remember seeing this previously.