What is Partial Close?

When executing a partial close, the closed portion of the trade is filled at the current market price at the time of execution. If you later close the remaining portion, it will be executed at the market price available at that later time.

Since market prices fluctuate continuously, each closing action (partial or full) may be executed at a different price. As a result:

The first partial close may be executed at one price.
The remaining position, when closed later, may be executed at another price.
Both executed prices are then averaged and displayed on the order record.