There is a lot of reasons for this, what @Berry78 said is correct but also...
Being obese for a long period time changes your body. Your body has to compensate for carrying the extra weight, your bones get denser (mine are 3x denser than the average person my age, don't worry they still don't weigh much) and you have more muscle mass. Knowledgeable Drs understand this. You can have a low body fat and be obese on the BMI chart, just because of muscle mass.
I'm 5'6 and I weigh in the mid 180s. I look like I weigh about 20 pounds less. This is because of a lot of loose skin and a lot of muscle mass.
I want to lose another 20 pounds but honestly I don't really need to, physically I am small enough. Losing more weight would just be for a healthy BMI//insurance purposes.
The Dr is giving you a realistic goal. All the goals I got from calculators etc were in 220 range. If I was lazy or ate the way most WLS post-op plans suggest I would have never got past 250. I got to the weight I achieved by eating a way that worked for me and by being super strict with myself to start with.
200 is realistic, 135 is obtainable if you want to. No Dr is going to tell you that though, because WLS won't get you to 135, only you can. WLS will probably get you to 200, maybe 185, the rest you are going to have to grind out yourself.