Great I can use my fashion design arts diploma here
Yes, clothing grading differs from one end to another.

Due to mass manufacturing of clothing, manufacturers sometimes 'cut edges' when it comes to using textile bolts. So a pair of pants that's graded at sz. 10 can be a sz. 6 or, it can be vise versa a sz. 10 could be marked as a sz. 6.

The world of small, medium, large, extra large is for convience rather then putting a tag of all the mesurements. These garments we're made to fit the a majority of people with ease (ie T-shirts), but it's not as good as using yourself as the decieding factor.

It's hard to say what the 'standard' is for industrial garment sizing because clothing manufacturers may have different standards from another.

Read:
Will Size 8 Dress Soon Be a 12?

There are size conversion charts out there or go look on the ISOhttp://www.iso.ch/iso/en/ISOOnline.frontpage

Just google it but I can't gurantee that the charts you'll look at very due to the type of textile, clothing, etc... to suit the purpose of the clothing.

Confused yet Well, there's a lot more I can get into but here is what I know

Everyone has unique proportions. Charts are used to give a manufacture an 'idea' what will fit the majority (target) of consumers possible under one 'umbrellla' size.

It is next to impossible (except if you make or have a personal tailor) that store bought clothing will be perfect. Tags are part of a numerical ordering system and give salespeople an edge to sell clothing to a customer by observation.

Sizing by garments lables alone is deceptive and should not be used be the main indicator of progress BUT using a measuring tape properly and comparing your mesurements to a standard chart for size will give you an idea what RANGE you sit comfortably in.
Also, clothing companies prefer even numbers as oppose to odd numbers for clothing. Only stores use the odd numbers (more sales) but it always looks like
6/7 or 10/11
Some will try on a sz. womens 10 jeans from five or six years ago and then go to a store down the street to try on sz. 10 to find out it doesn't fit, but the sz. 14 does! We change sizes, the industry does too.

My advice, since everyone must be dizzy from reading :

When your clothes don't fit properly, buy new or get them altered.
Altering is a bit expensive BUT they measure you out and can give you the measurments so now you have inches, rather then 'sz?'.

CUT OUT ALL TAG SIZES!! Burn 'em, give them to your spouse, make a quilt, JUST DON'T GET SUCKERED IN. This helped me greatly avoid the 'perfect size trap'.
It sounds dumb, but without the tags, you don't focus on the number but the the way clothing feels and fits, which IMHO is better. Get that monkey off your back and concentrate on the better part of your lifestyle change!

Finally, last but not least: Spandex blend clothing.

The greatest misconception. Spandex blends to me is the hidden enemy along with size tages. It's not all evil, it does make one look good but I would avoid wearing it when I go out to eat or wear spandex blend clothing every day (especially bottoms).
Spandex IMHO should be only for working out and not for everyday wear, except for undergarments. Even polyester pants with the elastic waist band is way better b/c it doesn't stretch everywhere, just at the waist!

A pair of spandex blend jeans/pants can be a womens '30' or 'sz 12', it's true until you put them on. It give allowance up to an inch to possibly 2 or 3.
It's hard to figure out if your lossing or gaining because it stretches.

Companies did this so they can outfit as many people as possible with one pair of jeans under one size (also cost effective for manufactures)

Best clothing to wear is the jeans w/o spandex allowance or pants with a proper zipper and button waist band.

But if your hung-up on the sizing thing, you'll have to use mid to upper labels (boutiques). Usually there closer to the standard sizing b/c there clothing is exclusive and it's based on 'quality' not 'quantity', thus, more expensive. Also, most of there clientel custom order, so the clothing on display are for display and for the sales associate to give the tailor the sz. so they can create custom block from existing blocks...blah blah blah blah..


ex: Lida Baday (very good sizing), Chanel, Armani etc...

So sorry for taking up all this space but fire away on my journal if you have any questions.
Were all working for the same cause and though those little hiccups are the small reminders/challenges, not the end of the world.