Some hr's do not appear

I have the style for a hr below:

hr {
    width : 100%;
    height: 1px;
    size:1px;
    background-color: rgb(0,0,0);
}

and I have 4 hr's on the page. The problem is that when I do

height: 1px;
size:1px;

or

height: 1.0px;
size:1.0px;

Some hr's appear.

But when I go to:

height: 1.1px;
size:1.1px;

On, then all hr's appear.

How do I fix this?

This can be seen here