Apparently size five in shoes boots are most popular so why don't the shops supermarkets stock enough of these sizes! You will notice on the "sale" shelf the sizes are 8, 9, shows you what is least called for!
Our local department had trendy boots on offer but when I asked for size 5 was told that product is finished!!
my daughter is a size 5, I never have trouble finding her shoes from anywhere in that size, maybe too many people buy them so they run out of stock before the least popular sizes
shoe fashion is very changeable, maybe they fear if they make too many they will be left with unsold stuff that is out of season, believe it or not they actually research consumer spending on shoes to estimate how much stock they should make give or take some penny pinching of course ;)