The answer by Ben is 100% correct they’re technically different and even though they have the same fit and the connector is the same look at the back of a S8 and S9 digitizer and lcd you will notice the difference of certain resistors and or capacitors and certain other things like if you look at the little gold dots near the bottom on the back they’re different. I’ve worked in shops for years and people make this mistake all the time when buying their own parts because the image online will show s8 screen when your trying to get a s9 screen. Even in the shops newer guys after 6 months of constant repairs would sometimes make the mistake but it can cause damage like shorts and even blow out chip/resistors/capacitors I have had it happen to people many of times so if you think you have one of the opposite screens on your phone and it works fine your most likely using the correct screen cause like he said technology is different and there’s subtle difference on the back like look at the placement of the gold dots arranged near the bottom on the back of the screen.