Where did you get the brightness rating for the S9 as 1000 cd/m2 (nits)? The S10 was tested at 1200, but the S9 should be around 540.
Record High Brightness Mode with Automatic Brightness On
On the Galaxy Note9 the Maximum Screen Brightness can go much higher when Automatic Brightness is turned On, so that users can’t permanently park the Manual Brightness slider to very high values, which would run down the battery quickly. High Screen Brightness is only needed for High Ambient Light, so turning Automatic Brightness On will provide better high ambient light screen visibility and also longer battery running time.
When Automatic Brightness is turned On, the Galaxy Note9 has a High Brightness Mode that produces between 710 cd/m2 and 1,050 cd/m2 (nits) in High Ambient Light, which is where high Brightness is really needed, and is considerably Brighter than with Manual Brightness when Automatic Brightness turned Off.
The 710 cd/m2 is for an All White Full Screen 100% Average Picture Level APL image, which is the most challenging image for an OLED display because all the pixels are at full power. This is 27% Brighter than on the Galaxy Note8, and a Contrast Rating for High Ambient Light that is 32% higher than on the Galaxy Note8 because of its lower Screen Reflectance of 4.4 percent. This is a Record Setting Brightness for OLED displays with 100% Average Picture Level APL images.
The Peak Brightness of 1,050 nits is less than the 1,240 nits on the Galaxy Note8, but that is only for very low APL, and there are very few Apps and images that take advantage of the higher Low APL Peak Luminance. See the
Brightness and Contrast, the
High Ambient Light and the
Screen Reflections sections for the measurements and details.