I routinely set Google Flight alerts for expensive flights so that if prices drop, I can cancel and rebook.

Planning a trip to SA and when comparing Google Flights and Matrix ITA to United.com I'm seeing flights/classes on United that are not showing up on Google or Matrix.

Example from JNB->SFO on Google or Matrix
Business & PE = Only LH options via FRA
Economy = Desired flight via EWR

Same route on United.com shows tickets for all 3 classes via EWR.

Now to make it even odder. Look at JNB to SBA in Biz and Matrix ITA conveniently connects you via EWR then DEN, LAX or SFO.