In "face select mode" in Edit Mode, I've selected some faces, and when I switch to "vertex select mode," it reveals that additional faces - which I do not want to select - are part of my selection. My selection appears to be accurate in "face select" mode. When I switch to "vertex select," it reveals the 'extra' faces selected. Then, if I switch back to face select mode, the extra faces remain selected.
This is expected behavior :))
Faces can share vertices, so switching to Vertex select and back can add faces to selection.