All prices in a backtest are read from historical data.

The prices printed at any bar are from the files that you set up with the "History" variable. If that was a .t8 file, it's the first, highest, lowest, and last underlying price of that bar.

The underlying price at which an option expires is the last underlying price of that particular option in the t8 file. As you can easily see in the log, it is not identical to the prices printed at bars.

So if you don't trust a certain price, look into that .t8 file and check the prices.