It's not the highest price bid. It's the highest price bid among the cheapest set of bids which could fill demand. Say we need 100 units of electricity in the next hour, and the bids are 50 units @ $2/unit, 30 units at $5, 30 units at $10, and 30 units at $20. Then, to get 100 units, you only need to buy from the first three suppliers, who are offering 110 units between them. The highest price among those is $10/unit. The 4th supplier's price is higher, but he's not among the winning bidders.
It's not the highest price bid. It's the highest price bid among the cheapest set of bids which could fill demand. Say we need 100 units of electricity in the next hour, and the bids are 50 units @ $2/unit, 30 units at $5, 30 units at $10, and 30 units at $20. Then, to get 100 units, you only need to buy from the first three suppliers, who are offering 110 units between them. The highest price among those is $10/unit. The 4th supplier's price is higher, but he's not among the winning bidders.