In this case the 'best' rate applies because the risk of fraud is much less. The rate only becomes a significant factor when a certain volume is achieved. Everything is relative and one should not base one's decision on only the absolute lowest rate possible. The business, in its deregulated state, is not static, is quite dynamic and changing processor's is not uncommon particularly when one's business grows to a point where leverage can be exercised in order to obtain better rates. They do vary from business to business or type of business.
Merchants play games too.
If they need a certain amount of funds they have the means to 'transmit' or generate funds through family, friends or foe alike. There is risk inherent in the system since it is not possible to verify all transactions at the point of sale. Time catches up to those who employ devious measures, but often after it is too late to recover. It is the conclusion of the resulting formula, a certain trade has more likehood of misuse than another. Now, back to the question at hand.