And I'll leave a full direct answer here as well.
At the current moment there's a desire to revise the cost of aircraft but we want to make changes having the basic data which helps us to understand the most comfortable prices. So we are conducting a research to receive these data to make decisions taking into account result of this research instead of making decision based on nothing. 15 aircraft were chosen for this research. Prices differ as that is the point of the current research. And before you ask, the answer is - yes, they are put randomly for fairness, e.g. there's no dependence on number of battles, amount of gold/money you spent, how old your account is, etc.
You need to be logged into the premium shop to see these offers. It might take time for the system to recognize you. All these aircraft are considered to be special offers technically, so there's no possibility to use a coupon (coupons are not applicable to special offers). We'll discuss this situation and prolong coupons when this research ends.