Snapchat best friends is calculated in a super simple way:
Basically, it is just a counter of which people you use Snapchat to communicate with the most.
Each Snapchat message sent or received is counted as 1 point, and the person who you get most points from, is your Snapchat "best friend"
How Snapchat best friends are different from real life best friends
The thing is, this isn't really an accurate way of telling who your real life best friend is.
For example, my best friend doesn't even use Snapchat. So my bestfriend on Snapchat is actually just the person I enjoy goofing around with on messaging apps with...
But if you want your best friend to be correct on Snapchat, all it mean is you guys need to send more Snapchat messages to each other!
Have fun :)