Because everyone has their own opinions and that is what makes the market.
If you expect the market to always be 100% right, then all equities would always fully valued, so why would you then bother investing in equities?
Who knows why I was able to buy SDG at less than 60c when its NTA was way over $1? Who knows why the DJS share price was only around $3 before a takeover proposal pushed it to clost to $4? People have different opinions and this is what makes the market. Some people will be right, some people will be wrong.