Anyone consider that the difference might have to do with the strings? For example, perhaps the strings gain slack while not in use and the linear density or string stiffness changes as a result.
hurrah, a very good point made at last. Strings do go off, and at different rates. Typically depending on the amount of playing. A pro will change strings around every 6 months, and get the bow rehaired annually - some will do it more frequently, depending on the strings. E.g. Evah Pirazzi strings will go off and sound really really dull after 3 months (for me anyway) - and they're pretty much the most expensive. They do, however, sound ace when new.