This is a scam, not sure why you are even confused by this.
The chance for Private Key generator to generate a collision (a previously owned address) are very slim, you have a beteter chance to win a lottery ticket than generating a collision address
Bitcoin has been around for 7-8 years now and there are never a reported collision, I doubt yours was too.
If there was a collision, then it means the wallet or website are using shitty number generators

p.s. scan your PC right now as the program might be a potential virus that could log or steal your data