My understanding is that the call sign is entered for FLT NBR on the INIT page (e.g. EZY8418), not the flight number (U28418). I have heard of reports of pilots mistakenly entering the flight number instead of the call sign, meaning that incorrect data is transmitted via ADSB.
Why would Airbus use such a misleading title?
The underlying info is covered really nicely in this article. Long story short, flight number was used for a long time until there were so many flights the repeated numbers and similar names became too lexicographically similar over the radio to the point of confusion. Governing bodies eventually switched to an alphanumeric call sign to avoid these issues.
To reduce the likelihood of call sign confusion resulting from call sign similarity, airlines and ANSPs turned to alphanumeric call signs. Alphanumeric call signs replace the formerly purely numeric call signs and are assigned to flights to in such a way as to maximize the differences between call signs and minimize the likelihood of confusion in a method called deconfliction.
Its simply a hold over of older verbiage in the display.
We also now live in a world where flight number may not be truly unique.
