The reason is that the next file number will jump to 1 plus the highest number found on the card or 1 plus the last file number shot taken by the camera, whichever is higher.
You can review jpegs from any Ricoh in any other Ricoh but just don't take a shot with that other card or you will have file number confusion.
The file number is the last 4 digits so files can be numbered from 1 to 9999 before something happens, like a warning of "folder full" or a new folder is automatically created to cause the file number to effectively progress from say R0009999.jpg to R0010001.jpg - but it's never that simple as the default seems to be to start a fresh camera always at R0010001.jpg as the initial folder created seems always (in my fiddling) to be DCIM/101RICOH/R0010001.jpg
But I notice that my R3 at least is up to DCIM/100RICOH/R3007451.jpg so it is operating back on the 100RICOH folders instead of the 101RICOH as it was when I bought it. Confusing. Maybe I did that.
Now the secret to getting the file number to look like R3xxxxxx or R4xxxxxx etc is to have a card, format it and then take one shot. Either remove the card to a card reader or use the camera connected as a USB device and then use Windows Explorer to rename that shot from say R0000123.jpg to R4000123.jpg then put the card back in the camera and take another shot. The next shot taken should be R4000124.jpg. But also check the folder number as that may have incremented as well, so you may have to rename the folder to 100RICOH or whatever you want and take another shot and see what happens. Format the card again and take another shot to see if the new numbering "stuck".
Sometimes it takes a few tries at formatting, shooting and renaming file numbers and folder numbers to get things to stick, but it appears to be stable from there on.
Before a long trip I like to reset the camera to 0001 so it's easier to keep track of how many shots taken. Later the camera can be set back to the proper count by examining the firmware page and seeing what the image count should be. I keep my photos in YYYY/MM/DD folders so there's no chance of two same numbers colliding anyway.
To reset to 1 you need to format a card, take one shot, rename that file to be effectively 9999 then take another shot and the file number should roll over to 0001 in a new folder incremented by 1. Format the card again and take one shot and then rename the folder back to 100RICOH if that's what you want, all shots now go from 0001 upwards and it's nice and tidy.
To get back to the real number you have to follow the procedure found in my page to see the firmware level, the bottom two numbers are...
SH: xxxxxxWith SH being the number of shots taken without flash and ST being the number taken with flash, add the two together for the number of shots taken by the camera. Then do the file rename thing based on that number to get back on track.
My R3 shows SH: 011954 and ST: 000951 for a total of 12,905 shots taken so far, so obviously I never reset my R3 after the last trip.
When I had the R5 and now have the R3 and R4, I can easily tell all the files apart because of that R3, R4, R5 prefix to the file numbers. When the cameras get to R10 in Feb/Mar 2009 I'm not sure what to do, but I'll worry about that when and if I ever buy one.