“Dimensional bleeding”. It’s such a fun phrase. It allows an author to get away with practically anything they want in their stories.
The term originated on the blog “Observe and Terminate”, a modern sci-fi story about a paramilitary organization (the PTC) which hunts supernatural threats (similar to the SPC). Dimensional bleeding is used as an explanation for the often contradictory reports which happen in the story (Basroil Squad being reported wiped out, only to come back alive and wondering wtf is going on with their death reports, and the contradictions between Observe and Terminate’s Solstice After Action Report and what zerosage later claimed to have happened).
According to the blog, dimensional bleeding is caused by Slender Man. Not much in the way of science explaining how he does it, beyond the essential “It’s Slender Man so he screws reality right up”. Whether the bleeding is intentionally caused by Slender Man, or if it’s just a side effect of his presence, depends on how powerful one is willing to make him. If you’re going for nigh omnipotent, playing around with dimensions Slendy, it would make sense for him to be able to cause the phenomenon. Alternatively, one could have a Slendy which alters reality simply through his complete alien nature.
Nor is there in depth scientific explanation explaining what dimensional bleeding is (which is forgivable, as the purpose of the blog is to entertain instead of provide scientific theory) beyond what can be inferred from the name. The idea seems based in the theory that there are multiple universes; which is the idea that there are an infinite (or near infinite) number of universes, each with a slight variation from the others. Bleeding between dimensions would mean that the parallel universes are affecting each other; events which would happen normally in only one happens in both, people display attributes from one dimension in another, etc.
What the inclusion of dimensional bleeding provides to the Mythos is a handy excuse to handwave inconsistencies across stories away. It allows for blogs and vlogs to share the same Slenderverse setting, without being bogged down by what every other story puts forth as their canon. So Story A claims that all of New York City was eaten by Slendy Man, but Story B has the protagonists wandering around the Big Apple without any problems? Then Story C shows Slendy riding a dinosaur? Not to worry! It’s just dimensional bleeding at work!
Course, within the setting, dimensional bleeding is kinda a bad thing. Thus the PTC has been devoting significant resources towards trying to fix the bleeding. One could say there’s been some success (we no longer appear to have people constantly posting contradictory posts on the blog), but it is still present, and thus still viable as a tool for explaining oddities in the Mythos.