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In your case i think the APs support preauthentication /PMK caching .

Preauthentication lets the MU authenticate to other APs over the wired network while its already asociated to an AP iff all the aps are in the same ESS and same vlan .

PMK caching :: .When you have two APs
configured for PMK caching .Initially the Mobile unit has to go through 802.1x to assocaite to AP 1 and when it roams to AP2 its has to perform a complete 802.1x again .[Now MU caches PMK for both AP 1 and AP2]

Now when MU roams back from AP2--->AP1 it need not go through the entire 802.1x exchange.One will be able to see only eapol hanshakes on the air.It will use the pmkid in the re-association requests which help avoiding the entire 802.1x exchange sequence.