Why do database host services increment id by > 1 unit? I have worked with various databases and notice that an id column (primary key) with auto_increment usually increases by a seemingly random, but static, value like 3, 5, or 10. 

So instead of the id counting up by one for each row: 1,2,3,4, etc. the id might count up by 5 like 1,6,11,16, etc.

Why is this practice in place?

My guess is that it can serve to obscure information to outside users.  For example my user id  might show in the URL on a website, and if the id on the users table counts up by 1, I'd be able to gauge the number of users that have signed up for a specific service.  However I am curious to know if that is the reason or if other factors are involved.
The only reason I can think of is "reserving" some data slots for the future usage. That would work if a table is index-organized ( or clustered in Microsoft terminology). For instance,  if we have rows with id 1,6,11,....1001, and we manually insert row with id =2 it will be physically stored (assuming data page is big enough to store 2 rows) in the same data page as row with id =1.
Personally, I doubt efficiency of such approach, but it seems to make some sense. 

Security explanation sounds very weird to me, increasing by 5 (or 3, or 103) is by no means more "secure" than by 1. 

There might be some very application specific reasons, but it's better to ask whoever designed particular application and db schema for it.