Why do some doctors say I need a tummy tuck while others don’t?

Just like anything else Plastic Surgeons have differing opinions on what procedure works best. Sometimes a tummy tuck would give the best result and sometimes the patient will only need to have liposuction. It depends on how much loose skin the patient has. I recommend that you go with the Surgeon that you feel the most comfortable with, not just the cheapest one. As in all things skill and results are what matter. I hope that this has helped. 

It sometimes comes down to a judgment call.  If your skin is loose usually a tummy tuck is the better option.  If you have good skin quality then liposuction can do the trick.  If you are somewhere in the middle it may work to do some skin tightening with Laser liposuction.  I would just get a few opinions then go with the surgeon that you feel the most comfortable with.
Good luck,
Dr. Tucker