10.5 is not 10 1/2 ... it is size 10, width fitting 5 (average). 4 is slim (women's shoes and boots), 6 is wide.
If your Stows were size 10 1/2 they would say 10 1/2.5
When you say that you have been wearing them for a while, how long do you mean? I used to wear a 10, I now wear an 11. Many people's feet spread a bit with age. But maybe you mean a shorter period than several years.
However, if you can show that your Chepstows are differently sized to one...