Keep in mind on UA we have "actual age" and "claimed age." When they announce boarding for families with children under 2, I see a lot of <2 yr olds that look much older boarding with their parents & extended families. Maybe a GA taking the boarding pass of someone carrying a child during pre-boarding assumes it's a lap child and only looks for one BP (bad decision, but possible). It's not unusual for inexperienced flyers to hand a GA both their current flight and connecting BP's, so that the GA just scans the one for that flight.

If a parent with a 2+ year old child preboards, perhaps that parent would be reticent to admit to an FA that the child is >2 years old, thereby further cementing the idea that the child is a lap child.

Just thinking of other possible considerations.