There is a big spread in processing times. It depends on how complicated your background check turns out to be, whether they need to do an interview, etc. Things like investigating to see if you are really divorced from a previous partner and investigating the 8 countries that you've lived in, etc., can take a long time. Straightforward applications can often be extremely fast, while the difficult cases can go over the wait time they list.
They used to list 3 numbers. I think it was 25% of applications, 50% and 75%, or something like that. The people whose applications were not in the first 25% used to phone constantly to see why their applications weren't done yet. Really, all it did was create false expectations.
Probabilities are complicated things that people don't understand well. If you tell someone that there's a 20% chance of rain tomorrow, many of them will conclude that it's going to rain tomorrow, when in actual fact, there is very little chance of it raining.
So, taking Moscow as an example, when they say 18 months, probably 75% of applications are spread out between 6 months and 18 months, with the average being somewhere in the middle. The other 25% of applications are completed in less than 6 months or more than 18 months.
By giving a figure of 18 months, they are trying to tell you not to keep pestering them because it's normally not productive to do that. If it comes to 18 months and you still haven't heard anything, there is something wrong and at that point you have reason to pester them. This leaves them free to work on your file without constantly being interrupted by questions from you. It's annoying to those of us who are waiting, but it makes for the most efficient process.
