Hi!
Looks like I'm slightly late to the party with this, but wanted to share my two cents anyway...
I actually work at a web design agency myself and we've written at least 3 blog articles about what bespoke website design is, means etc.
Often I point people to that if I am asked, however I still find a lot of people asking me the same sort of thing a lot and my next response is to compare the process to buying a pair of trainers.
If you go onto the Nike website and customise a pair of trainers, for example, you are not allowed the option of creating a shoe from scratch. You are only allowed to change laces, add names and make a few other small edits.
If you're looking to create a shoe from scratch, you have to be able to either call Nike directly, or source a specialist shoe maker to do it for you, where they'll take down all your details, ask you what material you want etc.
So I say that customisation is fine, but if you're looking for the real deal and you want something that has been made for only you that will last much longer, choose bespoke.