The Avalanche attempted counter found on the Run tab during test runs increments when a client SYN is sent. Stats are sent from the unit to the GUI at the sampling rate configured on the Run/Configure tab (default 4 sec). Based on the above in the case of unanswered SYNs (attempts) in-progress when stats are passed to the GUI there will be no data binned yet for unsuccessful, successful or aborted accounting for attempted counts higher than all values added up.
At test end the successful + unsuccessful + aborted Run stats should add up to total attempted.