It means that the subject being discussed is so unbelievable that "it never ceases to amaze me"
for example: The Tory party were voted into office by the British public for 18 years even though they deliberatly put 6 million people out of work, drove down wage levels to that of third world countries,and destroyed the National Health service.
It would be correct in these circumstances for someone to say
"it never ceases to amaze me just how stupid the voting public of Britain can be"