Unlevel bed to cab height

Recently I noticed that my bed wasn't level to my cab. The passenger side of my bed seems to be lower than my driver side. I know my trucks frame cant be bent due to never being in an accident and barely take it off roading through any rough or 3 wheel sections.

I did some measurements and noticed that my driver side bed panel is at least half an inch higher compared to my cabs bottom trim. Whereas on my passenger side, the bed panel is completely level to the cab trim. I should also mention that the points where to bed meets the frame on both side are completely flat making 100% contact to the surface.

The only two things that I can think that this could come from would be either: A) Tacoma lean due to the gas tank being partially attached to both the frame and the under side of the cab. However, when my truck is parked on a flat surface, my truck doesn't show any noticeable or significant lean towards my driver side looking at my truck straight on from the front and rear of my truck, even with a full tank. Or B) Uneven rubber cab mounts when the truck was first assembled. This being a more reasonable explanation.

I was hoping others with a 3rd gen might have noticed this issue with their rigs. I'd appreciate any helpful inputs or advice. I was thinking of just putting some washer between the bed and the frame in order to make things even lol.

 

I read a thread on TW recently on this topic. Maybe do a search? Hope you get it figured out.