Why do database host services increment id by > 1 unit?
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.