YNBU to not understand it, it is confusing, but common sense then tells you its relative to a few points - 1 being height & the other being do you carry fat or muscle, muscle being heavier, therefore you could well be over weight & very fit & healthy, as are a couple of muscle mary friends of mine - bodies to die for & technically obese
Dress size is the same, in that someone who was very small, say 4'10" & a size 16, would be perceived as overweight, & probably by quite a bit - though they could be a smaller size, muscle bound & tinier in size & weigh more.
I'm 5'5", not sure what I actually weigh now, but probably close to 10 stone - size 10/12 in general, but can sometimes need a 14 on top to fit my ffs in - yet I am a 32 in bra width, so my back is considered tiny.
FTW record, I am overweight in that I am over 2 sizes bigger than I was before becoming ill - sprang back to my old size 6/8 within hours of giving birth, I have always been naturally tiny & ate like a horse - I have a tiny frame & therefore even at a size 10/12 I carry more fat than I should, but asking my GP about the extra weight since becoming ill, it affects my mobility as a high proportion of my body weight on top of my old 7,1/2 - 8 stone weight - GP started suggesting I was anorexic

I'm not huge though & do like the extra curves, just not the extra strain on my already knackered legs.
So IMHO too, its all a bit daft & relative to individuals & shouldn't be taken as any indication of healthy weight - also it depends on where you shop - some designer brands I will still fit into a size 8, other cheap boutique type brands - I dread to thing - 