Weirdest Excel Problem?

hi - many times when opening excel file (my own or sent through company server) it seems to 'retrieve' an older version of that file - even if its not my own file - e.g., spreadsheet sent to me should have contained revisions I knew were made - it didn't - BUT the sender said it did - and day later when I went back and opened up the file again there were the right numbers ... At first I thought I was losing it - but I had printed it out so I know the numbers were incorrect the first time I opened it and now they are correct! Is this an excel program problem or maybe a server problem? Have never seen this so any help appreciated!