Why do some of my maximums sometimes decrease?

It is certainly possible for storage capacity to decrease. For example, compendiums provide a small boost to max science, so if you craft blueprints from your compendiums, you will see a decrease in max science.

I'm not aware of anything like that specific to oil...however, there is paragon, which boosts overall storage capacity, so you could arrive at the situation you describe by spending paragon points on Metaphysics...

Update: Another possibility to consider...your total storage capacity can decrease from crafting Tankers. This is because crafting Tankers consumes Ships, and Ships (with upgrades) provide a bonus multiplier to Harbour capacity.