The result of the shader's work varies. This is fine?

Question

OpenGL

I draw a texture in FBO with a shader and read the result. The vertex shader is trivial, and the fragment shader reads the pixels of the original texture (sometimes with coordinate transformation) and multiplies by constant matrices.
Problem: I noticed that if the input texture is static, then the result is slightly different each time (slightly different).
Is this normal and how can I get rid of it? I have a suspicion that this is because of the float, and all sorts of interpolations ....