When i take fish oil along with my usual dose of atenolol i notice my heart rate drops even more than with atenolol alone. why does this happen?

Confirmation bias: There is no mechanism for this. Most likely you are experiencing a deceptive phenomenon called confirmation bias, where a believed notion is confirmed as true since it is consciously monitored. When it does not hold true, the negative correlation is considered an anomaly and exception from the usual pre-conceived notion. Alternatively, taking the fish oil calms the HR by trusting its benefits.