Enter versus clicking submit button

In my form I have a few text fields (three textfields for SSN) and a submit
button.

<form method="post" target="viewFrame" onsubmit="return onSubmit()">

<input type="submit"
id="submit"
value="Search"
class="button"
onMouseOver="window.status='Click to search for documents'; return
true"
onMouseOut="window.status=''; return true">

In my onSubmit function - I set the focus on the first (of three) SSN
textfields. If I click on the submit button with my mouse - I get expected
behavior. If I click enter - I do not get the same behavior (I get the
focus on the 2nd ssn text field).

In my first ssn textfield:

onkeyup="nextControl(this, this.value)"

the nextControl function just moves the focus to the next ssn field.

Why am I getting different behavior when I click on the submit button versus
hitting the enter key?



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