The ABC is more cautious at the moment, basically. They haven't actually made any big calls already.
The AEC numbers you see (70 Coalition, 71 ALP) is just who is in front right now. The ABC has taken out the super close seats that could change.
However the AEC has a number of seats (4 now, was more earlier) that they have as not yet determined. The ABC adds those seats to their totals because they aren't actually in doubt, they just haven't had 2PP counting done - they're landslides on first preferences.
The AEC should actually show 73 Coalition, 72 ALP, 1 Green, 4 Independent. That is the actual breakdown of who is ahead right now. Its only a technicality that they're showing Labor ahead.