Underexposed pictures - Why?

Hey all, I shot my first roll of Ultramax 400 with an Olympus OM2n in auto or aperture mode and ISO set at 400 in overcast conditions.
Pictures 1) and 3) are the JPGs I got back from the lab, which are quite a bit underexposed. Pictures 2) and 4) are my Lightroom edits which lifted the shadows a bit and made the pictures warmer.
I wonder if the pics are underexposed due to how the lab processed them or e.g. because my is exposure wrong. I used the shutter speed indicator on left hand side of the viewfinder as a pointer but I might have done something wrong.
As a note, not all the scans are underexposed, but I'd say a good 50%.

https://preview.redd.it/nrm732opirfe1.jpg?width=3091&format=pjpg&auto=webp&s=e96d05eb9a04383e9c06c9f7276aa95c9207e582

https://preview.redd.it/enjze3pqirfe1.jpg?width=3091&format=pjpg&auto=webp&s=73edd89ca134ee960702c9bde468c55b5cef84d2

https://preview.redd.it/ce3omnwrirfe1.jpg?width=2048&format=pjpg&auto=webp&s=cf7c3bcc0a7e1243de2737509ba38011412ee690

https://preview.redd.it/r8zfyiosirfe1.jpg?width=2048&format=pjpg&auto=webp&s=665ab8dfba724a00f9ccc93f53b9d3ce80952d09