I've noticed the same thing too.

My theory is that they are using some upgrade method other than EUA.

As far as I can tell CO's EUA process upgrades on the lowest status of two passengers on a single PNR.

UA's UDU process upgrades on the highest status of two passengers on a single PNR.

My interpretation of the Companion Policy section of this webpage is that the CPU process will be the highest status of two passengers on a single PNR.