OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Computational Invariance

GLSL does not guarantee that two identical computations in different shaders will result in exactly the same value. The situation is no different than for computational applications executing on the CPU, where the choice of optimizations may result in tiny differences in results. These tiny errors may be an issue for multipass algorithms that expect positions to be computed exactly the same for each shader pass. GLSL has two methods for enforcing this type of invariance between shaders, using the invariant or precise keywords.

Both of these methods will cause computations done by the graphics device to create reproducibility (invariance) in results of the same expression. However, they do not help reproduce the same results ...