[bojan]

I, as a native Serbian speaker, have a 100% understanding of what is being said on Croatian national TV, and 100% understanding when reading websites in Croatian.

While the difference exists, calling it significant is just... untrue. For non-speakers, it is barely noticeable.

[mutation]

I have probably the same understanding of serbian. I used to read a lot of science fiction literature translated into it. Not to mention that I learnt cyrillic script in elementary school in the 1980's when the language was still officially called "serbo-croatian". But there are significant differences: for example serbian people don't understand kaykavian and refuse to. There are a lot of jokes on the theme in old yugoslavian tv shows. Kaykavian is part of croatian language.