Minecraft Version: 1.12.2
Forge Version: 14.23.0.2552
Mod Version: 0.1.0-70
Singleplayer/Multiplayer: Singleplayer
IndustrialCraft Version: 2.8.27-ex112

Additional Information:

• At first, I find this bug will happen only when a Creative Compressor connect with a Pneumatic Generator.
• First of all, the Pneumatic Generator and the Creative Compressor are both 0 bar.
• And then, add the pressure, both of the "+" buttons are okay. (For example, add to 8.0 bar. So sorry that I'm using Chinese.)
  
• Then, reduce the pressure by "-1" button. Here is the problem. The pressure is reduced to 7.2 bar but not 7.0 bar.
  
• Continue reducing pressure by "-1" button. The pressure is 6.2 bar. (Now the "-1" button is okay.)
  
• Reduce the pressure by "-0.1" button. The pressure is reduced to 5.9 bar but not 6.1 bar,
  
• Continue reducing pressure by "-0.1" button. The pressure is 5.8 bar. (Now the "-0.1" button is okay.)
  
• Reduce the pressure by "-1" button. The pressure is reduced to 5.0 bar but not 4.8 bar. (So it is bug again.) (And the bug will loop just like above.)
  
• Now add the pressure by "+1" button. The pressure is added to 5.8 bar but not 6.0 bar.
  
• Continue add pressure by "+1" button. The pressure is added to 6.8 bar. (Now the "+1" button is okay.)
  
• Now add pressure by "+0.1" button. The pressure is 6.9 bar. (It is okay.)
  
• Now add pressure by "+1" button. The pressure is 7.9 bar. (It is still okay.)
  
• I try to use other machines to reproduce this bug. And I find Elevator Base, Universal Sensor, Aerial Interface, Programmable Controller, Gas Lift, Thermopneumatic Processing Plant will have the similar situation.

So I was confused. Is this a bug or not?

Hmm, not sure if it's a bug, but I will do some testing on this. The creative compressor does work a little oddly compared with any other compressor.