Chavez had similar approval ratings at some point. He even won fair elections actually. I don't believe for a second that thee numbers are real. It might hard to hide a death, but not recognising it as a murder is actually pretty easy.
Chavez approval ratings could be tied to oil prices and declining poverty rates. As long as oil prices were high approval ratings followed. It worked until the collapse of oil prices.
If the numbers are so obviously fake, then why do even strident critics of the government not allege it?