We have had different experiences on different roads with different personnel in the booth. As I am sure you know there are many roads that are privately owned and not gov't highways. So they may have different formulas as to how they calculate the toll. I posted on the other thread that though we are mostly charged as a two axle bus towing a car (add them together) - we are sometimes charged as a 4 axle truck. I have no idea why. I am not even sure what differences there might be on which roads. Overall we find the calculator to be at least close to accurate and not a whole lot of difference from what it says to what we are charged. Though there may be some question about the charge we find it is not enough to worry about in the overall scheme of things as far as how much the entire trip costs us. We have had a few occasions when the toll cost less than we expected. As long as they are close to what we thought and the change matches the receipt we figure it must be correct.
A note on receipts - always save the receipt and be sure to get one - this is your proof of insurance should anything happen that the toll road owner is required to pay for due to road hazard or whatever. This gets into another whole topic of conversation but just thought I would mention the importance of keeping the receipt.