Not necessarily more serious - but most federal crimes are felonies and are punishable by over one year's imprisonment.

Common federal crimes are:

• Transporting a stolen vehicle across state lines.
• Making a false statement to the government with intent to defraud.
• Mailing matter that is obscene.
• Transporting or importing drugs.
• Forgery of government checks.
• Possession of stolen mail and of items - such as credit cards which have been stolen from the mail.
• Robbery or burglary of a bank or savings and loan institution.

In addition, the federal courts will punish violations of all state laws when committed on federal government property, such as a national park or a federal office building.

I thought a record could be sealed...

Yes a record can be sealed, either by law or by motion (Arizona Supreme Court Rule 123). But records, including juvenile records, are not automatically sealed.

Remember: a person convicted of a crime, either as a juvenile or an adult, has a criminal record for the rest of his or her life. Under some circumstances the juvenile adjudication may be set aside (ARS §8-348) or the juvenile court and department of juvenile corrections records may be destroyed (ARS §8-349). However, a record will still exist in the criminal justice system.