Because the rolls are not what you think should be random they are not random?
The fact that there is no pattern visible and numbers appear closer together than expected actually means it is more likely to be random.
Humans are not good at making random strings of numbers, because it doesn't "feel" random.
8 8 3 8 2 8 8 6
8 2 5 0 1 4 9 5
Both examples above are just as random, but the top one probably feels less random as the number 8 repeats itself more than you would expect.
Is there any means to prove whether the site is offering random dice games or not, I believe that there isn't any way to prove it and the best option here is either to stick with reputable gambling sites or just stop playing dice altogether. Dice itself is based on luck alone and maybe the player is having a very bad day.
The provably fair websites have the option to check your rolls on the blockchain.
Now most people won't do this as it takes some effort, but it should be possible if you read up on it.
