Why is html form with one textbox submitted after hitting enter using jquery, but not submitted if there are two?

Question

Both forms don't have submit button. Form A has two textboxes, Form B has only one. My question is : "Why doesn't Form A submit when hitting enter while Form B does?". I don't want to submit the form actually.

Form A

<button type="button" id="login-btn">LOG IN</button>
<button type="button" id="logout-btn">LOG OUT</button>

<div id="div_login_dialog" style="display: none;">
    <form id="form_login_dialog">
        <table>
            <tr>
                <td>
                    <p><label>How many Picklists?</label></p>
                </td>
            </tr>
            <tr>
                <td>
                    <input type="text" id="login-picklist">
                </td>
            </tr>
            <tr>
                <td>
                    <p><label>How many Items?</label></p>
                </td>
            </tr>
            <tr>
                <td>
                    <input type="text" id="login-item">
                </td>
            </tr>
        </table>
    </form>
</div>

Form B

<div id="div_logout_dialog" style="display: none;">
    <form id="form_logout_dialog">
        <table>
            <tr>
                <td>
                    <p><label>How many items didn't picked?</label></p>
                </td>
            </tr>
            <tr>
                <td>
                    <input type="text" id="logout-item">
                </td>
            </tr>
        </table>
    </form>
</div>

Jquery

$("#login-btn").click(function() {
        $("#div_login_dialog").dialog("open");
    });
    $("#logout-btn").click(function() {
        $("#div_logout_dialog").dialog("open");
    });