The grid lines that appear above staves when you’re in note input (the ones you’re seeing here) indicate rhythmic positions. In this case, each little tick shows where the demisemiquaver subdivisions of beats are, which is why when you move the B or input a new B at the next grid line after the A, it becomes a demisemiquaver.
The spacing of the bass staff quavers is being stretched because of the rhythm in the treble clef staff - in order to keep the notes together in rhythmic time, the visual distances between them varies. You might find that once you’ve input some more notes, this gap narrows because more material is filling the system and bringing the sextuplets on the top staff closer together. Or, you could change how much space is required for notes of different durations - that’s known as note spacing, which you can change (more info about those options here).