I don't understand what "each address is shared by nearly 294 private keys." means...
It means that there are mathematically an average of 294 different private keys that will all work with the same address.
Nobody will ever know more than 1 of those private keys, but logic dictates that on average there are that many other private keys that would work with the same address.
Each address is accessible by 294?
296, but yes they would be if you were able to calculate what any of those other private keys are. Realisitcally that will never happen, but if you are calculating the odds of a collision you need to include the possibility of colliding with ANY of those keys and not just the 1 key that you know.
When I export a private key from an address it only gives one private key to me.... 
Correct. Only one key will ever be known.