It seems that the Global Event tallies undercount delivered jobs, often even when connected to the internet, as well as when the connection is intermittent.
I've noticed this behavior and it seems to happen reliably on both the iOS and MacOS versions of the game. I've noticed it even when pretty much constantly connected to the internet. Here are the results of some recent experiments. The situation is that there is Global Event in a city X which a given Game Center account did not already have, which was added at the beginning of the event. Thus, all jobs delivered to the city were blue jobs and should count towards the event. Two recent experiments were that about 2/3 of the jobs counted towards the event: in one instance, there were 65 jobs counted in the global event whereas the airport info tab showed a total of 103. In the other, the numbers were strikingly similar: 67 out of 110. Those were both on MacOS with constant connection to the web. I generally overdo the 50 job minimum to ensure that I get credit, and figure that if I deliver jobs when away from a connection, at least I may get some of them to count. So I've seen lower fractions sometimes when playing some away from a connection.
There also seems to be a lag in when jobs are tallied- this under tabulation seems to be different from that. It's not unusual to deliver 20 jobs at first and have it read "will be available soon" or for it to read just 2 or 4 delivered, and then for it to jump up later.