Last time I crossed the San Ysidro Pedestrian border into the USA at 9:00 am on a Thursday, BWT indicated the line was 70 minutes. 120 minutes later I was facing the agent. Just before I faced the agent, I again checked BWT. It said the wait was 18 minutes. I looked behind me and clearly the wait inside the building alone exceeded 18 minutes and I knew there was a substantial number waiting outside the building. To add to my pain, the line next to me had three agents processing travelers. My line had one agent.
Someone in front of me complained to an agent that the line was too long. He replied, "It only took you 30 minutes or so, didn't it." Clueless.
The line cutters were out in force, offering to allow me to cut in line for $10. (Last time it was $20.) If you are not familiar, what these scammers do is put someone in line, then "sell" you their spot when they are near the first gate. Problem is, three or more travelers take the place of one scammer. Not a big impact, but quite unfair.
Just a little whine before dinner.
I had the reverse happen to me the last time I crossed back. I did it late afternoon on a Thursday. The online system said the wait was going to be 40 minutes. I had no wait whatsoever. Walked directly up to the agent. But I have no answer for your question. I assume someone at the border is just eyeballing it and giving an estimate.
I've never used a line cutter though. Might keep that in mind if I go again. Who do you see to pay the $20 ?