Tyranid organisms are always just particles of a greater being. Every hyve fleet is just a tentacle, and the small organisms scattered around the galaxies are more like spores costantly irradiated by it. You could even think of it as the scent particles of the hyve creature. Makes sense that if the tyranids are present in a galaxy close to yours, you will have the "scent" of it reaching your galaxy. Remember also that memories and data are flowing over the synaptic web, not through the immaterial like the messages of the astrophats. This means that they follow physical constraints. Something "felt" by the hyve fleet Behemoth can take years to be "felt" by the Leviathan fleet. Everything that we see about the behaviour of the hyve mind is not based on quick and istinctive reactions, the hyve always takes its time. It is inevitable, not fast. Leviathan may have absolutely no idea of what an Ultramarine looks like. Leviathan encountered a pesky opponent called Blood Angel on multiple worlds, and decided that it was better to just weed it out preemptively in one huge strike instead of keep fighting a guerilla battle with them on all the systems.
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