A shopper who wears three different sizes across three brands is not misjudging anything. Apparel sizing has no enforced standard, and the cost of that shows up most clearly in the economics of online clothing.
Sizes are drafted from a brand's own fit model
Every label builds its patterns around a fit model, a person whose measurements define the base size from which all others are graded.
Two brands using different fit models produce genuinely different garments under the same size label, and neither is incorrect because there is nothing to be incorrect against.
Older voluntary sizing standards exist but are not binding, and most brands departed from them decades ago in favour of a fit that suits their customer.
Grading multiplies the divergence
Sizes above and below the base are produced by grading rules that add or remove fixed amounts at each measurement point.
Bodies do not scale uniformly, so a garment that fits well at the base size fits progressively less well at the extremes of the range.
This is why the same brand can feel reliable in one size and unreliable two sizes away, and why the fit of an item depends on how far it sits from the pattern it was drafted for.
Returns are the direct consequence
Online apparel carries a return rate far above other categories, and fit is by a wide margin the most common reason given.
Each return costs a return label, inbound freight, inspection, pressing, repackaging and restocking, and the item spends weeks out of the sellable pool.
A meaningful share never returns to full-price stock at all, going instead to clearance or liquidation because the season has moved on while it was in transit.
The cost is priced into the garment
Retailers know their return rate by category before the season begins and set margins with it included.
That means the price paid by a shopper who keeps everything includes the cost of the returns made by everyone else, which is unavoidable in a category where fit cannot be verified before purchase.
It also explains why free returns are being narrowed or charged for, since the cost had grown faster than the margin covering it.
Why ordering two sizes is rational and expensive
Faced with unpredictable sizing, shoppers order multiple sizes intending to return the ones that do not fit, which is a sensible individual response.
In aggregate it doubles the handling cost of every order and raises the return rate the retailer prices against, so the practice funds itself through higher prices.
Detailed measurement charts, expressed in garment dimensions rather than size labels, are the only reliable way out of the loop, and they remain far less common than they should be.