Why would the seal, rubber ring, expand on the Master Cylinder cap?

I had the problem that I could not put the MC cap back on. I saw that the cap's round rubber seal/ring had stretched which prevented the cap from sitting correctly on the MC fluid container.

I purchased a new MC cap from Toyota and a couple of months later the new cap's rubber seal has also stretched. So now the cap is on at a slant.

What would cause this stretching of the seal?

Thanks for any assistance ...