Well, found a solution, if not an explanation:
scaling in the outer div instead of the image tag made it work.
Code:
<div class="img60">
<img alt="ad-subs" src="../Images/ad-subs.jpg" class="img100"/>
</div>
.img100 {width: 100%;}
.img60 {width: 60%;
margin-left: 20%;
margin-right: 20%;}
Both are identical in epub, but only this is as intended in AZW3.