Well obviously your Lucario isn't level 100 yet, and that will have some effect, but not a lot. The main things here are EVs and IVs. If you've killed more Pokemon that give off Atk EVs, it'll make Lucario's Atk stat higher than his other stats. Also, if his IVs are naturally higher in Atk and lower in SAtk, it'll result in a higher Atk stat.
In case you don''t know, here is an explanation of EVs, and here is an explanation of IVs :)
