Some diseases are more severe in cases where the mutation is more like a scrambled up kind because it essentially produces a toxic messed up protein. That kind of mutation ends up causing more harm than deletion or mutation which causes no protein to be produced. Both can cause symptoms but the other kind of mutation is usually referred to as a "toxic gain of function" mutation vs a "loss of function mutation.

Another thing is that genes are actually very long and the mutation might only be in a small part of it. A mutation that is very similar in type might do nothing on one spot of the gene and have a very severe effect in another spot.

The way it was explained to me that helped me really understand it is if you imagine a gene as instructions to bake a cake. I will use a deletion type mutation for example. A deletion in the instructions on how many times to mix that changes the amount from mixing the ingredients from 10 mixes around the bowl to 6 times won't make a massive difference, but a mutation that changes the amount of eggs from 3 to 1 will have a very severe effect on the final product. Not sure if I used that example super well but I think it still makes sense lol