Lower back arch

Hi everyone, I started new post because of my problem with lower back arching, just when sitting only on the ground with straight legs. I have no lower back pain, rather upper back tension when I try my best to sit straight.

I also have problem with archer lower back while deadliftin and squating too (while going ass to grass) - so I suppose all of my problems, which I listed, could be caused by my tight hamstrings, but I rather enclose photos and believe here is huge amount of people with experiences and advices :-) Thank you!

I have no idea, other than to say it looks structural as in perhaps your spine is just built differently?

I dont know, but It looks unnatural, like I have some bulge in my lower back and it quite annoys me. 

Looks totally normal as your tailbone is not straight down.  Keep up the hip mobility and core strength routines and you would be able to choose to rest on your lower spine as shown or arch your pelvis and straighten your back to remove that curve.