EMS 單number have 2 types, 1. Within China, 2. International.
As so many seller only have the EMS 單 within China, they may simply complete it and as EMS to collect. However, the said 單 will be revise by EMS afterward with a new number.
If you send goods to Hong Kong via EMS, most of the seller will make this mistake, and there is no need to worry.