Wall of Mouths

I have two less serious questions for the band if they so wish to answer I’d be most glad!

1. Why is there a B at the end of dumb but not at the end of dim?

and…

2. Why is instant custard, such as Birds, called ‘instant’ when it can be bought ready made in a tin? Surely the ready made stuff, such as Ambrosia, is the most instant you can possibly get?

Thanks lads :D