The answer to your question is actually quite simple, and the only issue was that you must of read it but you probably misunderstood what the meaning was.
"???'s sprite usually appears as three concentric circles alternating between white and black around a question mark in the center. Whereas index number 0000 and other ??? usually use Bulbasaur's footprint, there are a few which use other Pokémon's footprints." - Bulbapedia
Like I mentioned before in your other question about Decamarks (if you remember), Decamarks takes on many different forms of glitch Pokemon varying from game to game. This can also simply mean that Decamarks either has many different footprints that its able to take on, or the different versions of Decamarks has different footprints.
"There are many other variations of this Pokémon in the Generation III games. Their images are the same as ???'s, but the individual sprites can be backwards, differently colored, upside down, or any of several other minor differences in appearance." - Bulbapedia
"Footprints are the tracks left by a Pokémon. Each Pokémon has a unique footprint that is displayed in the Pokédex of every game since being introduced in Generation II. However, this is only applied to Pokémon with visible legs." - Bulbapedia
Also since Decamarks is a glitch Pokemon, in reality it shouldn't have footprints at all. It only has one because its a glitch Pokemon.
I hope this answers your question, feel free to comment if you need some more information on this.