Why does Facebook oAuth give different User IDs for different sites?

Question

Facebook

Actually there are 2 sites, each with its own theme, but it is supposed to provide users with the opportunity to use one personal account to pay for various services. Sites use oAuth registration/authorization. Everyone else except Facebook always gives the same User ID, but for some reason Facebook gives different User IDs for the same user.
For example, for my Facebook student for two different sites, the following User IDs are issued:

1. 10203901073150053
   
2. 10203804195928183
   

Here's how it will be settled, maybe I'm not taking the correct user ID? Is there a correct one somewhere?