My conclusion is, that he was shot, and then subsequently joined a different unit and was given a different number. Or she was sent information that was not related to her husband but rather a different Joe Smith. I hope perhaps someone could shed some light as to wether men were moved after wounding, and would this involve a new army number?
I have the following information.
Medals are stamped Pte Joseph Smith 13720 2nd Worcestershire Regt
Letter dated 30/10/1915 I regret to inform you... 13720 Pte J Smith admitted 22 field ambulance ... gunshot wounds...
Letter dated 18/08/1917 I regret to inform you... 31918 Pte Joe Smith
12 btn Hants admitted St Elmo Military Hosp, Malta... gunshot wounds both thighs and severe fractures...
There are countless other letters but I'll save you all the boredom

Hope someone can shed some light. many thanks