Attractions TMs - what exactly is the single rider policy when grouping odd numbers?

shoutout to any current or former TMs - I've got a question for you! First of all thanks y'all for your service, working through Easter and spring break is relentless and brutal.

I was in the park recently when single rider just *did not move at all* for an unusually long time. some people in single rider were literally jumping barriers to secretly cut into whichever line moved fastest. When I got to the front, I found out that one of the TMs doing grouping wasn't calling for single riders through the single rider line, but instead calling for any solos in the express line to come forward. So even if there was a 1 person queueing alone in express about 15/20mins away, they'd get brought to the front ahead of single rider. I asked the TM what the rationale was and she said her job was to prioritise express first, then normal guests, and single rider guests are also normal (aka. no express guests). so she would only use the single rider line if no one in express came forward.

this felt really strange to me - isn't the point of single rider to literally get single riders to fill single seats??? if they're just gonna keep finding solo riders in express queue, which of course is like infinite (considering anyone who wants an immediate ride can just volunteer to go ahead), then wouldn't it render all single riders useless?

so my question: what exactly is the single rider policy? Is there any value in opting for single rider if I'm waiting for a unicorn spot to open up where its a single seat unoccupied by both express and normal lines???