Clothing sizes

So I went looking for some cargo shorts today.  Looking for a sale or clearance prices on some good fabric.  I hate trying on clothing at the store.  So I guess big.  I like shorts to be baggy anyway.  I picked 36 waist thinking I'm at 34.  I want to get to 32 waist but I know I'm not there yet.  So I find the sale, fabric was crappy. Paper thin, synthetic, but they were not too bad.  I get home and try them on, oh my...they do not fit. Not even close.  Wow.  So now I'm depressed. If 36 s like that, that puts me in 40 to be baggy comfortable..  So I put them back in the bag then searched for shorts I had in bottom drawer.  I found I pair I've never worn because they are light tan and I tend to stain light shorts.  I out them on perfect fix, look at the tag. 34..  So how can shorts have such a drastic size swing?  Aren't sizes standard?  Is that not what determines a size?