Nomorobo actively maintains a very large database of active robocallers, but new numbers are being created all the time.
Background updates happen every hour on the hour, but in some cases spammers will allocate new "from" numbers and start spamming them after the last hourly download. When using the "Lookup" feature it checks the live database on the server which is up to date, hence the discrepancy.
Our system is very good at detecting these new numbers, but some calls will get through before these numbers are detected or reported.
Therefore in your case this number called your phone prior to the app knowing it was a robocaller, then our system subsequently detected the number and updated the app database, causing it to be identified correctly in the call history.