Web form overwriting fields that aren't even on the form?

As part of the lead import process, we have a "Number of Employees" field that gets populated.

When someone is registering for our Roadshow events using the landing page w/form on it, it is changing that number to 0, even though that variable isn't on the form at all.

More than likely it's user error - but what could be happening here? It's a pain to haave to go back and add that data to the records, and it's important details that our sales team needs to know.

Thanks!