Can you please explain this ?

Hi Guys,
so i play exclusively at B&M 7 days a week in evenings after my work. The limits at our casino are
2-4, 3-6, 4-8, 6-12, 9-18, 15-30 (there is no no-limit)

Out of 31 tables in the casino at most (meaning only on weekends when the weekend visitors come here) there are 2 tables for 6-12, 9-18 and 15-30 going. usually just 1 tables each, and rest are the lower limits

Currently I play at $4-$8 (on most days) and $6-$12 (on some days). While waiting for 3 hours yesterday i observed the $9-$18 game and took a shot. played very tight and aggressive and happy to anounce didnt lose my shirt. (won about 18 bb in 2 hours ).

here is my observation.
2-4 is soft game.
3-6 is a bit tougher (now I only played 40 hours total and that too when i was going through my first big downswing so observation might be a bit skewed).
4-8 is very soft.
6-12 is a lot tougher.

9-18 isnt that bad but i wouldnt call soft. a lot of 3 betting with shit, almost every alternate hand is raised preflop. people reraising with anything suited (so their hand is hidden..shit like that). basically 1-2 or higher at Party. and yes, people taking stab at the pot on the river with bluffs

Question 1: is there any real reason behind these games differing in toughness at alternate limits ? Like why is 2-4 and 4-8 so soft and not 3-6 or 6-12 .

Question 2: I am bankrolled for 9-18. but i dont intend to play it every day. Am i better off playing 4-8 full time building my bankroll, and taking shot at 9-18 once a week rather than what i do now (playing 4-8 full time and taking shot at 6-12 2 days a week). .

Question 3: how much do i need to "mix it up" playing this limit . I see people reraising with 7-8s, J-10s and trying to outflop the preflop raiser. I usually dont try any of this shit. its still ABC poker for me. Will I be too predictable and hence in trouble if i play it this way ? [/b]

9/18 or 10/20 is the border of where it turns into a people game instead of a card game from what I've seen.