Because I was a size 6 a few years ago. Then I found I had to buy a size 5.5 or 5 in most shoe shops, but thought that it was because I was buying different brands. However, today I tried on 4 pairs of chelsea boots in 2 different shops and the 5 was a little loose. Same with trainers.
Either my feet are shrinking with age, or shoe sizes are getting larger, and wider.