So let's say a 12 word mnemonic. The last word of the mnemonic is a checksum hash of the seed, which is the first 11 words (technically not entirely correct, some of the bytes in the last word are the checksum and some are part of the seed, but for understanding this you dont really need to know that, other than that this fact makes it easier to find the last word). A checksum is a number generated from data, that cannot be used to reverse engineer that date and can (almost) only get generated from the same exact data. Like a fingerprint of a document or something like that.
You can't (feasibly) do the checksum on paper. So you have to generate the first 11 words and then try a bunch of words for the last word in your signer and eventually it will have a valid seed.
Cheap dice... The less random the dice, the less secure the seed. But the wildly different, minor manufacturing defects combined with the unintentional randomness of how they are thrown make the dice sufficient for the most part. I personally use casino dice, but I'm also out of my gourd.
Do you have to be this level of paranoid? No. But not being this level of paranoid just cost 500 bitcoin wallets just under 600 bitcoin. Your dice are probably fine. What will help you sleep at night is not buying the right dice, especially after this incident. What will help you sleep at night is understanding thermodynamics as it relates to randomness and cryptography. Go forth and try to google your way into understanding why this dice roll shit works and you'll feel better.
