The "part of the month this bureau, part of the month that bureau" pattern was definitely fact (observed by multiple individuals checking their pulls). I don't remember where the explanation of why Citi was doing that came from. You'd have to find the many-years-old threads (probably from the days when 25k Citi AA cards were churnable at the rate of 2 at a time every couple months) to figure that out. (Ie, it would likely be a thread that predates CandymanJim's "the jig is up" thread from late 2009.)

There could be other factors besides price. There could be some relationship between certain banks and certain bureaus, for example, because someone high up at one went to work at the other. It could be that depending on what computer system the bank uses and what computer system a particular bureau uses, some banks interface better to EXP while others interface better to TU. It could be not that EQ has poor prices but that they are slower to return data from pulls than EXP or TU, so banks hate using them for that reason. Who knows. (I don't know any of these reasons to be factual, I'm just pointing out how many different possiblities there are for how this decision could be made.)

But the point is that every bank makes this decisiion individually. They don't care whether they're using the same bureau as another bank or not. You care collectively, but the banks' decisions are not made collectively. And since no one except churners cares about this issue, you're not likely to change bank's minds. But the point is they're not likely doing it to make it hard for churners (we're just too small a part of their customer base to make big decisions like this based on that), they're undoutedly doing it for one of any number of other "simpler" business and/or technical reasons.