There is definitely an algorithm and the outcome is probably what you present, however you have not considered the parameters.
From reading above and what I've heard from other players I suggest the parameters are similar to these:
Group - You Don't Matter - these players purchase diamonds consistently - no need to incentivize with offers
Group - you 've purchased diamonds it the past - these players have made an occasional 49.99 below purchases' - an occasional offer will incentivize additional purchases
Group - I really really want you to buy diamonds - these players have never purchased diamonds - rain offers on the player until a purchase is made
Sub group A - made a large diamond purchase - After the luxury of using this vast bucket of diamonds - this player will be hooked move them to Group - you don't matter - never another offer
Sub group B - made a small diamond purchase - The diamonds were nice but .... - move them to group "You've purchased diamonds in the past" - send the occasional offer
Likely there are a few other parameters, but there is no way they are randomly sending out offers - based on what multiples, alphabet.. what? A truly random algorithm would result in all players receiving special offers at random times - but for the player above to go years without ever receiving an offer suggest specific parameters not randomization.
This is my opinion anyway