@Spandey:

That's what happening with USCIS. I am not sure if it is a deliberate act of randomizing things or if there is another number (not shared with us) and if the processing happens in that order.
But, whatever it is it's not a first come first serve basis. I have heard that putting inquiry actually delays the case; so more desperate applicant are even more penalized - there is mixed opinion on this but USCIS officer can do anything at will.

Also there is no way you can argue that other cases before you are approved but not yours, as they will have the chance to counter you by saying that your have been stalking or breaching someone's privacy.

Also, I have to assure that two cases with consecutive number are actually received and sent receipt on the same day - I know this because you can add other IOE number in your profile and see the soft updates too. Lol. I am not sure if USCIS know this but this hack really seems to work to check what is happening to other cases around you.

So, we are at the mercy of random chances here - definitely no first come first serve basis.