Right. Bass modes aren't just left to right and front to back, they are also vertical. Another common thing bass is reinforced near walls and so almost always more full and round or even boomy, and gets tighter and more punchy as you move away, so standing up is also moving you away from the boomy floor bass towards the middle of the room tight punchy bass.
Why does bass have more “punch” when I stand up vs. sitting?
I have Rockport Avior speakers and I notice when I stand up the bass is slightly louder and has a little more punch than when I am sitting. This is with the same distance from the speakers. And the speakers are level with the floor. I do like the sound more when standing. Has anyone experienced this? Suggestions as to how to have the bass response while sitting as I do when standing?