Those are most probably in-flight residual packets that are denied because the connection on the ASA is already considered closed - so it is treated as a "new session" and hits the default deny in the ACL.
You can enable the "sysopt connection timewait" to have the ASA linger the deleted connections for a while (20 sec), then the syslogs should be gone.
Indeed this will cause a slight increase in the number of connections as seen by "show conn".
If you prefer, you can verify that my assumption is correct by following:
1) collect two captures on inside + outside
2) correlate the captures with the syslogs to see which exactly packets would be dropped.
What you should see in the captures is the very last packet(s) being denied.
