When a record is ordered sealed the Order the judge signs typically contains a list of government agencies that are supposed to be notified to seal the records. The entry in your case may not have included an agency that made the disclosure, thus they were never notified to seal the record. It is also possible that the agency was notified but failed to properly seal or completely remove the record from public view.
Another possibility is there are now companies that basically data mine court records for information The acquire information on cases as the case is filed. They may not remove the information from their private database when a case is sealed. If your employer will tell you who provided them with the information, you may be able to contact them and dispute the information by claiming it is not accurate. The company may then try to reexamine the court documents, only to discover they no longer are available. This may be enough to get them to remove the information so it no longer effects you.
You may want to consider contacting a criminal defense lawyer, who is familiar with the court that granted the expungement and see if they can follow up with Clerk of Courts toi make sure everyone was properly notified regarding the expungement.