uber anti tank units

[Aetius]

sometimes i encounter vehicles in a trench/bunker and don't find it very difficult to hit them.... it doesn't seem that the trench/bunker offer the same protection to a vehicle as infantry .. am i right?? it shouldn't protect ... how can i fit a tank inside a bunker ???

I think the designers felt the same as you. From the manual (around page 41, my emphasis):

Bunkers - Bunkers are the next step up in Fortification development. They cannot be created during game play, rather they start in place at the beginning of a scenario. What Bunkers offer over Trenches is generally timber protection from Direct Fire and overhead protection from Indirect or Artillery Fire. As such, Bunkers only protect non-vehicular units and are treated as Hard Targets in terms of their defense.

Pillboxes - Like Bunkers, Pillboxes are fortifications that cannot be created during game play. They also only benefit non-vehicular units and represent a much more permanent defense complex made of concrete and steel. Therefore units protected by these fortifications are considered Hard Targets.

Vehicle units do benefit from improved and trench hexes, on the other hand, which also makes sense.