A bug with some <div> markups?

Hi, I installed WordPress 2.1 and soon noticed the following behaviour, quite difficult to see because the “visual” result in a browser very often is right though the xhtml code becomes invalid.
When inserting some markups in a page, either from the editor or as the result of some plugin (for instance the wp-contact-form) containing some <div> which contain some other markups like <img> or <form>, here is what happens.
<div><img src= … /></div> (correctly nested)
becomes
<div><p><img src= … /></div> (not correctly nested)

I must say that it still happens even if there is no carriage return at the points where the <p> is automatically inserted.
In some other cases I also saw the ending </p> appear in the result of my displayed page with no beginning <p>, for instance my wp-contact-form puts a </form> at the end of its job, and looking at the source code of the result, I see </p></form> with no starting <p>.

I looked around in the options; I disabled the feature for correcting XHTML from the editor though it has very certainly nothing to do, since the trouble appears “on the fly” at display time.

Why?