We don't know. Apparently, the US Post Office reuses tracking numbers for international shipments. We e-mail you the tracking number that is printed on your package, created by the post office's API.
A quote from Shipwire: "USPS has been known to reuse old tracking numbers from previous shipments dating back a couple of years. The tracking information will update within one or two business days. USPS assigns tracking numbers to shipments and this cannot not be avoided."