Some women gain weight slower than others. I never gained weight during my pregnancy, only lost. Total I dropped around 25 pounds, and managed to gain back some of that.
No it's not true I lost over a stone in the first 28 weeks and then gained a stone so really I never gained anything in my pregnancy, the less weight you gain the better for you really :p you won't stay fat afterwards, your baby still will get everything it needs from you regardless of your weight gain!
It is going to vary for every woman. I put on a TON of weight during my first pregnancy, even when I was eating healthy and walking, I retained a lot of water, I am hoping this time around it is less. Some women will only gain 20 lbs. the whole of their pregnancy. Some women get a lot of morning sickness so they don't gain much, some women go crazy and eat everything and put on a lot of weight. If you start out underweight the Dr. might want you to put on more, if you start out overweight they hope you gain less. Don't worry about what others tell you, as long as you are being healthy, if you are worried ask your Dr. they are usually pretty up front if they think you are gaining too much or not enough.