HSRP - Failback from ISDN to ADSL

I have 2 x routers - 1 x 877 ADSL (HSRP priority 105) & 801 ISDN router (HSRP priority default of 100).

I failed over the connection to the 801 and believe that I saw the following:

1) The 803 becomes the active router - ping responses to remote site = fine 2) On fail back - the 877 ADSL port was plugged back in (NB I was tracking the ATM) 3) After a few seconds the 877 pre-empts and takes back up the active HSRP address 4) The pings continue to respond even though the 801 is now standby and the routing table on the 877 hasn't converged

How could this be ?

It may be that the traffic was going out the ISDN and returning via the ADSL. Can anyone substantiate that ISDN would still forward the traffic even if it had given up the HSRP priority ?

Regards

Darren