This might be clearer if you display nonprinting characters by pressing the
¶ button on the toolbar. (Press it again to remove the display.) Each tab
character is represented by a right-pointing arrow, as shown at
http://word.mvps.org/FAQs/Formatting/SettingTabs.htm.
Because you inserted a numbered list entry, you got a tab character between
the '1.' and the first 'TEXT'. Before you define any explicit tab stops,
Word assumes a default tab stop every 0.5" (or whatever you set in the
"Default tab stops" dropdown in the Tabs dialog). So at that time the first
'TEXT' starts at the 0.5" position.
Now you manually define an explicit tab stop at 5.5", and a side effect of
that is that Word ignores all the default tab stops to the left of 5.5" (but
it still honors the ones to the right of that point). The tab character
between the '1.' and the first 'TEXT' now pushes out to the 5.5" tab stop.
In addition, because the 'TEXT' doesn't fit between the 5.5" position and
the right margin, Word pushes it down to the next line (that's a little
strange, but reproducible).
What you need to do is define a regular tab at or near the 0.5" position to
"eat" the first tab character, and then define the tab stop with the leader
at the 5.5" position. You can define the regular tab in the dialog (don't
forget to click the Set button!), or simply by clicking on the ruler at the
0.5" mark.