This is just how the randomizer works. There is no deeper logic behind it.

But Tura pickups have consistent pictures -- all of them have the same color variant.

Pure accident. There's also just 2 or so Tura skins, while FIA has 8.

The randomizer has nothing to do with this. Editor gets preview pictures from vehicle config editorPreview property.
Guerrilla pickups (of any side) have these pictures:

• general pickup: "\lxRF\editorpreviews_rf\Data\CfgVehicles\B_G_Pickup_rf.jpg"
• pickup with fuel tank: "\lxRF\editorpreviews_rf\Data\CfgVehicles\B_G_Pickup_fuel_rf.jpg"

Tura pickups (of any side) have these pictures:

• general pickup: "\lxRF\editorpreviews_rf\Data\CfgVehicles\I_Tura_Pickup_01_RF.jpg".
• pickup with fuel tank: "\lxRF\editorpreviews_rf\Data\CfgVehicles\I_Tura_Pickup_fuel_rf.jpg".

You can use this code to make sure that pictures are static:

_control = (call BIS_fnc_displayMission) ctrlCreate ["ctrlStaticPictureKeepAspect", -1];

_control ctrlSetPosition [0.25, 0.375, 0.5, 0.25];
_control ctrlSetText "\lxRF\editorpreviews_rf\Data\CfgVehicles\B_G_Pickup_fuel_rf.jpg";

_control ctrlCommit 0;

The image creation is automated. It will spawn a car with random livery and make a screenshot of it. Thus, the image is random. Look at vanilla vehicles, they also show random textures, same as e.g. FIA soldiers showing random headgear. If any vehicle lineup has the same texture visible, it was by accident and not on purpose.

Understood. Thanks for the explanation.
But maybe it makes sense to recreate preview picture for pickup with fuel tank since it's so different from the general pickup?