Guess My Size

Can someone please explain to me why the sizing for baby clothes varies SO much?? I've known this about baby clothes for quite some time now, but it seems to be getting even harder to keep my daughter clothed! She is in 18 month clothes right now. Sort of. Some 18 month clothes are teeny and haven't fit her for months. Others are huge and she fits in them like a circus tent. And then other 18 month clothes fit her perfectly! Huh?? How does this make sense? And how am I supposed to know what size she is when there are so many variables involved? Grrrrr. The biggest problem is its not even just from brand to brand. If I could trust a brand to have their sizing the same across the board then I would know she is in 12 month in this brand, 18 month in that one, and 2T in the other. I bought Kalianna two new pairs of pj's a couple weeks ago. Same brand, from the same store, purchased at the same time. One of them fits her, but is a little big with room to grow. The other pair is so tight I had to actually pop some of the seams in the sleeves in order to get her arms into them. What the heck?!? And it drives me super crazy that the height and weight limits written on the tag are never accurate! Too often I have bought something that said on the tag 20-25 lbs (perfect!) and height between 29-32" (awesome!) Kalianna weighs 21 lbs and is 31" so this onesie should fit perfectly, right? "Ha ha! We fooled you! We switched the tags around, so this is really for a 9 month old!" If I could just trust the tags, instead of sizing by month, everything would be fine and dandy. Because I know my daughter's stats. Instead, its like the baby clothes companies have started a new game show: Guess My Size! The object is to figure out what size clothes your child should wear, and if you get it wrong, you're just out the money. "Ha ha! Fooled you again!"