Icons of beta sites are different form the icons given in the http://stackauth.com/1.0/sites . As example:  
<blockquote>
Original icon of the <a href="https://english.stackexchange.com/">English Language and Usage</a> is<br />
&emsp; &emsp; <img src="https://i.sstatic.net/827yM.png" width="30" height="30" />
<br><br>
where <a href="http://stackauth.com/1.0/sites">http://stackauth.com/1.0/sites</a> gives the icon<br />
&emsp; &emsp; <img src="https://i.sstatic.net/XpyJu.png" width="30" height="30" />

</blockquote>

Why?