jp,
An underscore character may have different widths depending on the font. What, exactly are you trying to achieve by using this as the leader, and how did you set the tab?
First, would underlining the tab work instead of using the leader? I do this frequently when creating a form. It's relatively quick and the line is always continuous if you want it solid, plus you have the option to change the weight, color and position, as well as the style of the line, by using the underline options.
Second, are you setting an ordinary right-aligned tab at the right margin? This can be inaccurate unless you do it "by the numbers" and if you change the width of the frame your tab is no longer where you want it. As an alternative, use a right-indent tab (shift + tab) which will always go to the right limit of the paragraph, regardless of changes in text insets, right indents, or frame adjustments.
Peter