How often does the Seller have to service the third machine?

Seller sold Buyer a machine. The contract provided that Seller would service it regularly. Seller services it every week. Seller then sold Buyer another similar machine with the same contract terms, and later sold a third machine. Immediately after the sale of the third machine, Seller discovered that in the trade, these machines are serviced only monthly. Seller told Buyer that because of that practice, it would be servicicing the third machine monthly rather than weekly.