TL;DL: SCP-4987, as well as SCP-4739 have conflicting descriptions in the tales and in the wiki. This is probably because the writer of the tales didn't create entries for them or picked a random number which was already taken cause who cares. However, I prefer to think that, due to their antimemetic properties, their files were corrupted or lost, and the SCPs themselves forgotten about, leading to the numbers being reassigned to other anomalous phenomena.
SCP-4987's database entry. It is described as a phenomenon where if enough humans and/or cetaceans look north at the same time, it will have an effect on the magnetic field, reversing it, and putting everyone who was facing north into a comma. It's a short file, you can give it a read. It has no memetic or antimemetic properties.
We need to talk about fifty-five, one of the tales that includes SCP-4987. It's described as an antimemetic scp that follows Marion Wheeler around, eating part of her memories every once in a while. She's fully aware of it and has it somewhat controlled, creating mundane memories for it to eat so that it doesn't take anything important. It appears in later tales in the same series as well, but the description remains the same.
Now, I would assume this is just the author choosing a random number for an scp, and not writing the entry for it for whatever reason, and then later that number is taken by another writer unknowingly to write their scp. Or even the scp already existing but the author going "whatever". I'd assume one of these is what happened. HOWEVER, there is a more interesting, in universe possibility.
SCP-4987 is an SCP with antimemetic properties. SCPs with such properties are not very well known, and regularly forgotten about. Furthermore, it's an SCP with very little impact. It affects Marion Wheeler personally, later on it affects one other person. But overall that's it. It doesn't present a world-class threat, in it's current configuration, it represents no real threat to humanity or the Foundation. So there's not much reason to keep much track of it. Thus, I present the following idea:
SCP-4987 as described in the tales exists, however its antimemetic properties and little overall impact means that it isn't recorded, or its file deteriorated and was lost due to its antimemetic properties. Then, the Foundation, unaware that the 4987 id was taken, granted it to the current SCP-4987 .
I just remembered Alastair Grey, or SCP-4739. In the database, it is an anomalous jug of milk, that kills or attacks whoever attempts to open it, possibly preventing observers from seeing the attack itself. In the tales, it's an antimemetic SCP that takes the shape of a well dressed man. You can only see it once it's eaten you and is in the process of digesting you, leaving you with precious little time to attempt to defend yourself. It wipes most of site 41 before being defeated. Now, this one presents a similar structure, though it is more of a threat. In this case its antimemetic properties extend to recorded information about SCP-4739. It has a file, but it can only be read once it has eaten you and is in the process of digesting you. So again, the file is effectively inexistent, and the SCP forgotten about, so the number was given to some other SCP instead.