Lifetime Mining Contracts

I've seen some advertisements for "lifetime mining vouchers" on a website called CEX.IO, I'm not at all familiar with the website but I'm interested in the idea of purchasing a set gh/s for life I just don't know if it's what I think it is...  Here's the listing:

What I'm expecting is 24/7 cloud mining forever, but that seems a little too good to be true, am I missing something here and what are your thoughts on this investment? 

If I understand correctly you will get a set gh/s but the difficulty will continue to rise so you investment will be worth less and less over time.