Measuring height?

A couple of weeks ago, when I got my BMI measured at the mall (via BMI-meter? lol.. I'm not sure what it's called, but that's what i'll call it. You know, it's like a scale type thingy, and you stand on it and it tells you your weight, height and BMI), and according to that, I am 5'6" tall. I had my sneakers on, which would probably add .5 of an inch to my height.

When people estimate/guess my height, they say I look about 5'5/6.

But when I just measured it myself, I got 5'3. Why such a big difference? To measure my height, I put a flat surface (book) against the top of my head and made a mark right where it touched the wall. Then I used a ruler to measure the distance between my mark and the floor. I was very careful not to make my measurement overlap, and was as accurate as possible.

So do you think the "BMI-meter" was inaccurate? Or should I trust it and say it was my mistake even though i triple checked it? Those 3 inches do make a difference

its possible that u may have tilted the book without realizing it when u measured urself... also, u may not have been standing completely straight.  u should have someone else measure you to get a more accurate number