Well natures and stuff can only be determined when you can examine the game's code or through testing it repeatedly. If I had to guess, its likely someone submitted the Sandshrew to Serebii and showed its OT and everything, which would explain the lack of nature. I guess they took it as fact and printed it.
It should be said that it could also have been an oversight. A lot of people don't know that some events have a set IV number in (a) certain stat(s). For older events in generations one through three though, where there wasn't as much of a die-hard fanbase at the time, there is little data left of what those numbers might've been, if they existed at all. And now there is no easy way to find out either. That's why Bulbapedia also lacks movesets for some older events still.