There is a difference between a variable type charge, and unexpectedly large charges; the boundary being a bit subjective but possible to resolve in most disputes. Variable charge means that your fare costs 50$ or 60$ depending on traffic/route; but if you reasonably expect the fare to be $50 and they charge you $350, without making it explicitly clear that the fares are such, then it may be a reason for dispute.

For example, a somewhat common scenario for such chargebacks is seedy clubs charging $1000 per drink to tourists. The fact that they had that tourist drink in their menu doesn't change that it's both unreasonable and unexpected, and according to mastercard/visa agreements chargebacks may be possible.

In general, the credit card companies are motivated to protect cardholders, so their agreements with merchants sometimes put stricter conditions than local consumer right legislation - the merchant can either alter their behavior; or pay chargebacks; or not accept those creditcards.