This used to happen to me quite a bit out of BWI: after boarding, a PA announcement asking various pax to ring the call button. An FA explained to me once this was to ensure all checked in passengers were accounted for— specifically, those who checked in online. A GA once complained there was some related computer issue, but I also know that on occasion, especially for busier flights, they wouldn't even bother trying to scan the OLCI printout, so this was a quick way of matching boarded pax to collected receipts.

The computers were also blamed for two separate occasions when I lost seat assignments on upgraded tickets. I selected my F seat online when I checked in online, but evidently the GAs saw my seat as still available and gave it away. I was told if I changed seats during OLCI always to double-check with the GA (which seemed to me to defeat a main purpose of OLCI).

I last flew on an upgraded ticket out of BWI in Sept. 2007, and didn't experience any problems then, so maybe whatever the problem was got fixed.