For the Blu-ray versions they might have applied various amounts of blur effects (smoothing out of grain) or sophisticated noise removal algorithms on the scan results.
Moreover it is likely that they put more effort in digitizing movies which promised (in their eyes) higher revenue. So that's why decades-long classics might be prioritized over more recent movies. I also wonder if older movies might only be selected for digitization if their quality is sufficient while they weren't that picky with later movies? I wouldn't rule out a certain bias, however this is speculative.
And of course, grain depends on source material. A format like 16mm is smaller than 35mm and to get to HD you have to magnify the 16mm more than 35mm and thereby grain is more visible. And there are naturally differences between film stocks of the same format if prodcued by different manufacturers.
But we are not done yet. The ISO speed and exposure when filming plays a role too. Higher ISO means larger grains. Low light means the grain is more easily visible (esp. in shadows). Also prolonging time during film development (called pushing) makes grain more noticable. There are long-term trends regarding lighting. Earlier, esp. in Hollywood, movies were generously lit, so on average such movies show less low light issues. Horror movies on the other hand tend to show a lot of dark, sparsely lit scenes in recent times.
Psycho is in black and white. Scream is in colour. The film stock is quite different, the develop process is different, the chemicals are different. Generally speaking colour film is more difficult to handle making it more likely that grain is more pronounced here.
Most of those factors work not so much in favour of the 90s movies on average. Maybe that's why you see noise/grain more pronounced in digitized 90s movies. However with a different movie selection the result might be another. It depends how much influence all those factors have on the movies you select. Plus a more recent digitization will enable taking advantage of recent and better noise removal technologies.