I have two different accounts, with one city each, on two different realms, so that I can't cheat and send KP or goods between cities. One account (the one with a city on Winyandor) gets a bonus diamond offer about once a month, ranging from 40% to 100%. My other account (on Felyndral) has never had a bonus diamond offer (that I can recall anyway). Both accounts were created within 3 days of each other in December 2017. I have played both accounts evenly and they're at a similar level of development (55K score and 60K score) - That was the point of having two accounts, so that I could be logged into both cities at once using two different browsers.
It's either some kind of long-term experiment by Inno to try and gauge if average earnings are higher with or without bonus offers and one of my accounts is in the 'control group' that gets no bonus offers, or they've taken a bribe from some mega rich player on Felyndral to block offers to slow down anyone trying to catch up to them!
It does always make me chuckle when I see a new offer pop up on the wrong city, I would buy diamonds with a good (60%+) offer on my Felyndral city, but I only ever get offers on the Winyandor city and I've already bought enough diamonds on it that I'm comfortable for now, so they're missing out on my money - I am a man of extreme willpower and I will not buy diamonds without a good offer!
I just don't think it's worth it otherwise, too few diamonds for too much money.
