Using OpenGL GLSL 3.3 in a 2.1 context

I have an old Nvidia card (Go GeForce 7400) which I know supports only OpenGL 2.1. Even if I force GLFW to create a 3.3 context on this machine, the program is guaranteed to crash or returns a NULL window. However, I found the machine can actually run some simple shaders written in GLSL version 3.3 under a 2.1 context. They even have #version 330 core version declarations. To my suprise, the program did not complain while compiling these shaders and everything seems to work.

So here is what I did:

Create a context using GLFW and specifically passed 2.1 as version number.

Load and compile shaders written in GLSL 3.3, and everything works like magic.

To my understanding, GLSL 3.3 comes only with OpenGL 3.3+. And before that, the GLSL version for OpenGL 2.1 is actually 1.20. So can anyone tell me what happened? Is this because my graphic card's driver has ``secretly'' implemented GLSL 3.3 but supports only OpenGL 2.1 APIs? What is the chance to find this kind of weird combination to work in general? I'm pretty sure I shouldn't assume GLSL 3.3 to work in a 2.1 context in genral, should I?

BTW I think I am using Linux Nvidia proprietary driver for the card.

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