I have no doubt that the doctor got the right number---their measurement system is actually fairly ingeniously simple. You simply lay the baby down on the disposable paper that gets pulled out to cover the examination table, mark a line tangent to the feet, and then mark up on top of the head. With good hands and a compliant baby, getting those two marks right is easy. Then measure between the marks, and the length of any normally growing baby is enough relative to likely sideways displacement that any distortion from angle is likely to be quite small (my quick-and-dirty estimate is that at Harriet's height, a 1-inch sideways displacement should give less than 1% error). It's imperfect, but pretty damned good.
So, what about our earlier measurement of 30 inches? As with most surprising experimental results, it boils down to simple experimental error. Not quite so dramatic as accidentally finding particles moving faster than the speed of light, but then we're dealing with a much smaller scale and more poorly controlled experiment.
What could have caused it? Remember, Harriet was playing with the level, so we weren't exactly dealing with a stable instrument. I certainly didn't look to see whether the level was actually level, so it may have been leaning somewhat. I may also have suffered from some degree of an optical illusion since I was looking downward, with first Harriet and then the level further from me. I may have counted some of her fluffy hair without realizing it. She was being partially supported by me, as she worked on her great (and slightly premature) ambition of standing, so she may well have been stretching upward in some way. At the end of the day though, if an error doesn't persist, it's probably not worth trying to investigate its causes, since they are likely to be transient and, frankly, boring.
So, an interesting lesson in the banality of experimental error and the importance of proper metrology (which appears to be one of my current favorite scientific concepts, thanks in no little part to my ongoing work in synthetic biology). With regards to the measurement of babies, however, in the end the same judgement applies: we have a long trim baby (though she's coming more into standardized proportion, which of course doesn't mean a damned thing).