Retail odd footwear are normally single shoes which have become separated from their pair.
This can happen during internet sales when customers order footwear and which are sent back without the correct packaging or possibly in the wrong box, if they’ve ordered two different sizes to try on.
Retail odds can also occur in retail shops when customers have tried on shoes and they have been put away carelessly.
Or on the shop floor they have been moved around and miss matched.
Large companies and retail brands can have huge volumes of single or odd footwear.