How is it possible that the same WIF Private Key generates two different addresses?
The same private key can be used to derive a couple of different addresses. For example, there will be a different address generated depending on whether you use a compressed or uncompressed public key in the derivation. 

More relevant to your question, you can use a private key to derive a P2SH-wrapped segwit address (3ASaGJ.. in your example), or just a plain old P2PKH address (1C8ju3.. in your example). Both addresses are spendable using the same private key, since they are both derived from the same private key. 

Note that you should not design a wallet to create segwit addresses using an uncompressed public key (use the compressed key only!), due to potential backwards-compatibility issues in the future.

This image from the bitcoin wiki Address page helps to illustrate this point: