font size always nest when should be replaced

I'm using fckeditor 2.5 and viewing with Linux firefox 2.0.0.8.

1. select size "smaller" and type a word.
2. select size "larger" and type a word.

The resulting HTML is not what I expected:

<p><span style="font-size: smaller;">smaller</span></p>
<p><span style="font-size: smaller;"><span style="font-size: larger;">larger</span></span></p>

In the 2nd <p> smaller is still in effect when larger was chosen.

1. select size xx-smaller and type a word
2. select size x-smaller and type a work
3. repeat for small and larger

The resulting HTML is also not what I expect:
<p><span style="font-size: xx-small;">xx-small</span></p>
<p><span style="font-size: xx-small;"><span style="font-size: x-small;">x-small</span></span></p>
<p><span style="font-size: small;">small</span></p>
<p><span style="font-size: small;"><span style="font-size: medium;">medium</span></span></p>
<p><span style="font-size: medium;"><span style="font-size: small;"><span style="font-size: large;">large</span></span></span></p>

I think this is a bug. Should I file a issue in trac?