Sometimes people's early roles are not credited. Or sometimes they are, but under a different name such as when they haven't fixed on their permanent screen-credit name yet. Or when they're embarrassed in advance over a low-budget, low-quality item they did to get groceries, and use a fake name. The databases don't always pick these instances up right away. Sometimes they come out hilariously as a film clip when they appear on a talk show, bad hair/makeup and all.

~S~