I noticed that too but didn't think too much about them and tucked them into the gap. My initial thought is they might be used to restrain the bunk ladder from sliding around while under way by tying it to the heavy mattress platform.
Our ladder stays put shoved way up in front, buried behind the tote of kid toys and (lately) pile of Halloween decorations but I imagine for safety reasons the ladder should be secured (and I shouldn't have all that crap sitting up there!).
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