Two pairs of shoes marked the same size regularly fit differently. The reason is that the number describes one dimension of a three-dimensional object, and imprecisely at that.
The number encodes length and little else
Shoe sizing systems assign numbers to length increments, with a small fixed step between consecutive sizes. Width appears only as an occasional letter, and depth is not encoded at all.
Feet vary in instep height, forefoot width, heel width and arch length, and two feet of identical length can differ substantially across all four.
A single length number therefore cannot specify fit, which is why the same size feels correct in one shoe and unwearable in another.
The last is what actually determines fit
Shoes are built around a last, a three-dimensional form representing the foot shape the maker designs for. Every shoe from that last inherits its proportions.
Manufacturers develop their own lasts, so brands acquire consistent fit characteristics that regular customers learn through experience rather than from labeling.
Because the last is a design decision, a brand fitting one foot shape well will fit a different shape badly regardless of which size is chosen.
Women's ranges are often scaled from men's
In several categories, particularly athletic and work footwear, women's shoes have historically been produced by scaling down a men's last rather than designing separately.
Foot proportions differ on average, including relative heel width and forefoot dimensions, so a proportional reduction does not reproduce the correct shape.
Purpose-built women's lasts have become more common, but the practice varies by brand and category, and labeling rarely states which approach was used.
Width options have narrowed at retail
Width fittings once existed across mainstream retail. Carrying multiple widths multiplies inventory for each style, and retailers reduced the range to control that cost.
The remaining assortment concentrates on a middle width, which fits the largest share of customers and leaves narrow and wide feet with fewer options.
Online retail has partially restored access to specialist widths, though it substitutes the problem of buying without trying on.
Fit problems have consequences beyond comfort
Persistent pressure, rubbing and altered gait from poorly fitting footwear can contribute to foot problems over time, particularly with daily wear.
Foot dimensions also change across life, including during pregnancy and with age, so a size that fit for years may no longer be correct.
Ongoing foot pain, numbness or deformity is a reason to see a podiatrist rather than to continue adjusting shoe choice independently.