The forum doesn’t decide what the various smileys looks like. They’re actually emoji, and the vendor of your computer or phone decide what they look like. For example, the unhappy face, which is actually Unicode (a system for representing all the various symbols and characters in a myriad of languages) code point U+1F626 “Frowning Face With Open Mouth”, looks like this on an Apple platform:
whereas you see this on Windows:
There are other emoji that might fit the bill of “unhappy face” better, like the “Frowning Face” emoji. It would look somewhat like this, with variations depending on your OS and web browser:
As for the smileys not showing sometimes, that’s problem of OS and application developers for not updating their software to properly display the full Unicode character set, which includes the emoji this site uses. While Apple platforms and Android are generally good at displaying emoji properly, Windows and browsers like Google Chrome don’t always implement these things correctly, often resulting in broken or improper representation of non-English texts and emoji.