Suzeeq mentioned blocking: this can significantly change the appearance of an item. If you don't know how to do it, search on the Internet for it-- there are plenty of tutorials. Almost every photo you see of a finished item is the result of blocking and good lighting. I've had things I've photographed for Ravelry turn out much worse OR much better than they look in real life. Lace in particular can look very, very different once blocked.
The other factor is that if you don't use the same yarn they suggest (I almost never do), it will come out differently. Even if it's the same gauge. Because of the way the yarn is spun, it can drape differently, or feel different, or give better or worse stitch definition.
As for keeping gauge in your projects-- try knitting a much bigger swatch and then you'll get a better idea of what your gauge is going to be. There are people who check constantly as they knit something, and constantly change their needle size as they go, because stress or being calm can change their gauge.
Here's another suggestion-- try just practicing a certain sort of garment, something small such as mittens, hats, socks, or scarves. Let's say mittens. Try just making cuffs. Make 6 of them. See what is going right and wrong with them. Then, try the beginning of the hand. Then the thumb gores. Then the rest of the hand, then the decreases and ending the tops, then the thumbs. Do each section on all 6 samples. I bet by the time you finish the 6th, you're going to get much more of an understanding of where you might be going wrong, and what you're already good at.