When those two things aren't in alignment, you can get a feeling of eye-strain. The way the lenses bend the light make it so the focal length that you're focusing on is further than the screen in front of you, so that isn't what is causing the eye strain.
Mind you, this is just my layman's understanding of the subject, so definitely take it with a grain of salt.
[0] - https://en.wikipedia.org/wiki/Vergence-accommodation_conflic...
So you’re not focusing on the glass, but on the image at that virtual depth.
The strain comes from looking at things rendered further or closer than 1.3m.