How do Chinese aliexpress seller use Singapore Post?

As we all know, there is only one postal company in each country, but I usually buy things from Aliexpress China stores. There are serveral different shipping options as Singapore post, China post, Austrian Post, Bpost, HongKong Post Air Mail, Swiss post and so on. So the question is, why they can do that?

Agencies of different postal companies
They don't have post office in China. But, there are many International Freight Forwarding Agent Companies in China. The seller can turn their product to those agencies. And then, the buy can still track their product in the website of different postal website.
Though Hong Kong
Another explanation is they  all send to Hong Kong, then transfer from hong kong to other countries, cause hong kong is a free trade port.

Why do people choose singapore post?
Because it is cheap, as you know if you choose DHL, FEDEX,TNT and UPS, the price is even more expensive than the price of that product. So they choose postal service, and China post is not always the best choice for different destination countries. Besides, singapore post can transit small parcel with batteries.