You are correct that properly implemented encryption cannot be decrypted without having the password.
Such decrypters as you see on that site don't actually try every possibility to get the password (known as "brute forcing") but they look for other vulnerabilities to decrypt the files of a specific ransomware.
Some of these ransomwares are poorly written and allow an expert to look at the code and find a way to decrypt without knowing the password. Some are so badly written the key to decryption can be found in the code itself.
The reason why some ransomware can be decrypted without the password is the same reason why not every encryption program out there is good: creating a program that encrypts securely is quite difficult, as some of these ransomware makers are now finding out.