Why are some tummy tuck lines so straight while others are like a zig-zag or not even? Is it the surgeon or the person's shape?

There was a time when Dr. Pauline Regnault taught tummy tuck techniques and introduced the W incision. Despite what Dr. Regnault saw as advantages, the aesthetics of the scar leave much to be desired. Dr. Regnault felt that putting the scar in the pubic hair (1975) kept the mons from being pulled up.  She also felt that the higher incision in the midline made it possible in all cases to remove the belly button donor site hole without having to close it as a small vertical. Her design was intended to put more tension laterally (which I think is a very good idea) and so equalize the midline and lateral tension. The upper flap usually has a longer skin edge than the lower, so the shape of the W made the length of the lower skin edge more equal to the upper, thus decreasing skin folds from the length discrepancy. Regnault P, Abdominoplasty by the W technique. Plast Reconstr Surg 1975; 55: 265. 
Despite these supposed advantages, times have changed. Less tension is present in the midline with advancement of the flap with progressive tension sutures, and with careful measurement and redraping, the upper and lower flaps can usually be brought  together with  any excess length of the upper worked in laterally.
Women today often do not have pubic hair, so the hairline is not as important as a smooth, pleasing and low hidden scar that does not rise higher than a small bikini. 

Thank you for this excellent question.  The more tummy tucks a surgeon does, in general, the neater the scars are.  However, there is not a single plastic surgeon around who does tummy tucks who hasn't had an uneven scar.  Sometimes it's the surgeon, sometimes it's the patient's anatomy.  For instance, uneven excess loose skin on the anterior thighs can often make the ultimate scar asymmetric because as you pull the abdomen down, you pull the anterior thigh up.   In my practice, I try to have the scar 2 finger breadths above the vagina, across the pelvic area, and then follow the crease between the abdomen and thigh.  I intentionally put tension between the mons pubis area and the abdominal flap to pull the vulva upward, decreasing or eliminating the stress incontinence.  To accomplish this, I flex the surgical table to 30 degrees to create this tension.  It is my belief, along with many other plastic surgeons, that the scar should be low and thin.  A beautiful scar above the bikini or panty line is still a scar.  The scar your plastic surgeon creates often can make or break the surgery.  However, a well hidden scar trumps a beautiful scar any day.

I make my incisions for a tummy tuck based on how patients wear there underwear.  If a patient's underwear tends to be straight across, I'll make my incision straight across.  I'll use a "W" type incision when patients like to wear their underwear real low.