Because Win98SE still allows some DOS access by booting to command prompt and DOS is much more lenient with filenaming using extended ASCII characters (128-255), files named with characters in that set are legal locally and there are quite a few characters that will show up as "_" when they are actually something else. Windows networking though tends to work with the ASCII set of characters from 0-127 and treats the remaining characters as some kind of placeholder that it cannot interpret. This is why you see filename differences between the network version and local version. To see the true filename, go to a command prompt and look at the filename.
As an aside, when in college, I used this trick to allow me to put files onto a windows fileserver from DOS because Windows could not delete the files since Windows deletes files via a separate call to the "delete" command for each file, but since the filenames map differently from DOS to Windows, each attempt to delete the file in Windows failed with a "File not found" message because Windows was trying to remove a file that did not exist as far as the FAT was concerned.
To demonstrate this, go to a command prompt and type:
Type test >> (alt-255).txt
Hold the left Alt key and type 255 on the numeric keypad then release Alt to get the character above. After hitting enter, you will have a text file wih the contents 'test', but which Windows 98 has no clue how to handle the filename for. In DOS, the filename will look like a space, but like anyone that has dealt with DOS knows, spaces are not allowed in filenames using DOS 6.22 or earlier or using DOS 7.x outside of Windows.