Add dynamic parts in texts

I really can't understand. I picked this example, so I have this in my jsp file:

<s:text name="msg.error">
    <s:param >Event01</s:param>
</s:text>
<br />
<s:text name="name.msg.param" >
    <s:param >mkyong</s:param>
</s:text>

and this in my .properties:

msg.error = This event doesn't exist: {0}
name.msg.param = This is a message from properties file - param : {0}

But the result is:

This event does not exist: {0}
This is a message from properties file - param : mkyong

I do not manage to find a real difference.

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