I have used two different rice cooker measuring cups, and I know the kitchen scale is correct, but the rice packaging for various rices say 45g = 1/4 cup, but if I fill it up to 1/4 cup, the weight of the rice turns out to be 30g. What am I doing wrong? I even put a little more than 1/4 cup but still it's like 32-33g. I know different grains weight different, but I think 15g off is too much right?
The measuring cups that come with rice cookers are not the same size as a US cup (or indeed many other countries' cup measurements).
A rice cooker cup is 180 ml, a US cup 240ml, and in a lot of places it's 250ml. This is because rice cooker designs originate from Japan.
You're getting 30g, which is 2/3 of what you expect. For a US cup, 3/4 would be right, and for a 250 ml cup, 72%. That's probably close enough given how much variability there is in packing rice into a cup, especially as grain shapes vary.