if I remember correctly... the only valid lengths for WEP passwords are 40, 64 and 104 hexadecimal combinations. so the ASCII lengths allowable are 5, 8 or 13. (I'm a little fuzzy about the 40hex/5ascii part, but the others are correct)

could it be that the other wlan client you use is using the "pass-phrase" method to generate the hexidecimal setting from the 7 digit password?