Texture/Tgatool2a question

[FKTrains]

Hello,

In screenshoot in attachment (from Open Rails), 2 identical shapes using a different texture. Let's call them texture A & texture B.

The left one uses texture A taken from an existing route.
The right one uses texture B. To create texture B, I open texture A in Tgatool2a, I don't change anything at the image or its alpha channel and I do an export with type "Ace (DTX1)".

As you can see on the screenshot, the 2 textures gnerate a different rendering.

Has anyone any idea how I can generate a texture B which is identical to texture A ?

(I have try to play with the options available in Tgatool2a under the alpha channel (Include Mipmaps, use mwdds for DXT,...), but without success...)

Ty