score:8
The I-94 automation process rolled out in 2013. Before that, passengers were given paper forms, and not all entries were tracked the same way. Five years is just the broad guideline of how long the website is able to provide data, but the website is only as good as the underlying database, and that database is far from perfect.
In short, there's nothing really to worry about. You entered before the latest system was up and running.