Measure string seems returning wrong first character width!

I've been coding a new usercontrol textbox. (I've my own reasons)

Everything is ok, except one:

It seems that, the first character of any string, was wrongly measured, even I use Graphics.MeasureString or Textrenderer.MeasureText. But I'll use results from Measuretext method. For example, the character "I" in Segoe UI font, size 28, has the width of 10 pixels. But when I measure this string: "II" (double I), measuretext method returns 42 (!), instead of 20. So is there any mistakes? Or there are extended spaces between characters? (I don't think so because triple "I" returns 52 pixel width)

Here is the usercontrol (not completed). You can see, if you type in, the first character always have more width than others same character, so it makes the caret move further than it was.

Call me Zenocoding