To expand on what was already a good answer, the Higgs Boson itself is not long-lived enough to interact with any of the ATLAS detectors. It decays into several other particles, which also decay until you're left with a bunch of less exotic particles.
Since a whole bunch of things are happening at once, all shooting particles at your detectors, you get a whole mess of particles all at once, and it's kind of a pain to separate which bunches of particles go together. So what they're saying is that the detectors have been seeing bunches of particles that look like they might have come from decaying Higgs Bosons, but there's still some chance that some other bunch of particles could have have been decaying in just the right places to look like decaying Higgs Bosons.
Since a whole bunch of things are happening at once, all shooting particles at your detectors, you get a whole mess of particles all at once, and it's kind of a pain to separate which bunches of particles go together. So what they're saying is that the detectors have been seeing bunches of particles that look like they might have come from decaying Higgs Bosons, but there's still some chance that some other bunch of particles could have have been decaying in just the right places to look like decaying Higgs Bosons.