The actual graphic (colored pixels) in your texture is not the same size and/or position as in the default texture. Otherwise yours would not get cut off.
https://wow.gamepedia.com/Viewing_Bl...interface_code
-or-
https://wow.gamepedia.com/API_Texture_SetTexCoord
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh