I am glad Paul has posted - he is far more knowledgeable on these matters than I could ever hope to be.

Having said that, three little thoughts did come to me -

Not all whites are white. Is it possible that the colour is (subtly) different?
Is the thread a different material? Polyester or silk, rather than wool?
Is the thread a different weight? Say 16oz, rather than 13oz?

That's it - I have no more.

Regards

Chas