Well usually it would be odd for a size 10 foot to fit in a size 13 boot..but who knows. sometimes the sizes are way off. But anyways were the boots used? odds are they are fitted to someone elses foot. When you first get boots they slowly change and packout according to your foot.
Anyways since you already bought them you minds well try them out as long as there not sloppy. If they get uncomfortable go to a shop and try on as many different pairs as possible to you find that perfect fit.