If I had to guess, I would say that this image was underexposed and then corrected for (perhaps unwittingly) when scanning.

Two reasons for this guess:

1. You mention you metered using the camera's built in reflective meter. I believe the FM2 uses centre weighing for its metering. That means that it determines the exposure mainly on the light in the centre of the image. In this case, that is a big, bright window. The camera meters this light, adjusts for it and exposes for that light. However, looking at your scan it would seem you wanted to expose for the room indoors instead, not of whatever is visible through the window. Since your camera exposed for the bright window, the darker scene indoors should be largely underexposed. When scanning, or when editing, correcting for such underexposure does not do the grain any good.
2. You are using a flatbed scanner, which by itself already increases grain on film scans. Especially combined with my first point, I am not surprised to see more grain than was to be expected. Generally speaking, you can use a DSLR to scan or scan darkroom prints in order to get sharper, less grainy digital copies.

As for you scanning at a PPI of 3200: the Epson V600 only has a true measured resolution of 1560 PPI. I don't know what interpolation may do to the grain, but this could be a factor too.