seconds_behind_master flips to 0 & spikes back, when running show slaves status

we setup maria 10.0.4 for multisource replication.

it was replicating from a percona 5.5 master. When we started replication, maria was a day behind its master.

its working fine but we noticed this phenomenon:

when we execute 'show all slaves status' consecutively the 'seconds_behind_master' flips from 0 to XXX and XXX to 0. XXX keeps decreasing though, which is good as its getting closer to the state of its master.

But wanted to know why the 'seconds_behind_master' comes down to 0 then again sets to a positive number.

Note: XXX is some positive number.

Elena Stepanova asked to file a bug for the same.

bugs related to this issue
--------------------------------
http://bugs.mysql.com/bug.php?id=52166
http://bugs.mysql.com/bug.php?id=53167

these were fixed in 5.6. We need to either merge the bugfixes or fix them on our own