I think he means that you buy a (used) car for £xx,000 and insure it for £xx,000 but find that you are only covered for a lesser amount even from day 1.
I had a used car many years ago that was written off after a couple of months, the first of several offers were way below what I had paid and thought I had insured it for.
Eventually the AA legal team got me a figure much closer to my purchase price.
Also when you renew, does your insurance company give you any indication of what value they are offering cover against?
I doubt it, they just quote a premium and do not tell you what amount of cover that is for!!
OK, we all understand that it is current value but a naive person would think they would get their money back!!