Perhaps there are a lot of other tiny factors involved in this particular issue (blurring):
maybe the paper is not bone dry,
maybe the glass is thermally expanding and taking the neg with it,
maybe the wooden back is expanding and taking the paper with it,
maybe the different coefficients of the various materials used in the frame are warping it over the extended printing period and allowing less than perfect pressure?
Maybe there was just a smudge on the glass?
Who knows for sure? It's interesting trying to reason it out, though!