The women in the royal family probably have more style hacks than most, especially as they have to spend several hours on their feet and be seen and photographed in public a lot of the time.
For example, Meghan Markle wears shoes that are ta size too big, and the Queen has someone to break in new shoes for her. This is so they will not have any blisters, and apparently Kate Middleton has her own shoe trick to avoid those too.
According to HELLO!, she buys her shoes it two different sizes, between a UK 5.5 and a 6, depending on the brand.
For example, she orders her Jimmy Choo shoes in a 38.5 (she wears those a lot at evening dos), and Rupert Sanderson shoes in a 38.
However, there could be another explanation, as Italian designer shoes often run big, so she might order those in a smaller size than she would for her favourite UK brands, such as L.K.Bennett and Russell & Bromley.
It will also depend on the style of shoe she goes for, as strappy sandals are more likely to rub, and on the season and location, as in summer or in a hot country you would need to factor in swollen feet.
The Duchess is also known to favour insoles to cushion the soles of her feet.