My first digital camera (Canon PowerShot G1, Christmas gift in 2000) used 100CANON, 101CANON, et cetera, and changed folders every 100 photos. This was all on a classic FAT-formatted card, so the folder names were only eight characters long, and the file names were eight-dot-three format. The 100-file limit probably has the same derivation.
My wife's current Canon (not new, but the handiest one to grab and check) uses a different pattern within the DCIM folder: 123___07, 124___10, 125___04 on the current memory card -- same increasing serial number prefix but an inscrutable variable suffix pattern. Still holding to eight characters. Looks like that camera starts a new folder after 384 files but continues file and folder numbering across memory cards and erasures.
Canon adapted the convention to its own needs at some point. So did Nikon and Canon. Although they have diverged, they retain enough of the common basics that they are still recognizable as camera cards or cameras by other devices.