Please don't get sucked into the 'normal' vortex.
Like the rest of the human race babies are individual and these statements are guidelines.
My eldest was so efficient at rolling that she had very little interest in sitting for ages. She could get anything she wanted, and that was her motivation- toys to play with, then she liked a baby doorway swing, then she was a quick walker. Her brother was frustrated by rolling, he preferred to sit and survey his kingdom, and took to the bum-shuffle to attain his goals.
Luckily I had a brilliant Health Visitor one of the old school -she was there to help you, not make you doubt yourself.
Of course if your baby is well outside the guidelines, one would take heed but for now I would not worry at all!