Hey, @waynejeong
Hoping you are excellent and having happy holidays.
Just adding, the official NXP Portal might be confusing, because it considers Flash Memory such as real Flash Memory + FlexNVM (Non-Volatile Flash Memory), and the part number that you mentioned has a FlexNVM of 128KB, so together the regular flash and FlexNVM, form 256KB.
Please check this pdf on page 18 where you are able to see this information:
https://www.nxp.com/docs/en/product-selector-guide/KINETISKMCUSELGD.pdf
Let me know if you have more questions.
Regards.
Pablo Avalos.