The traffic numbers reported in the Organic Traffic chart shown in Site Explorer can either be based on Monthly Volume or Average Volume depending on what you select in the dropdown:

Average Volume is based on the last known 12 months of data. Traffic that is based on Average Volume is more consistent and helps you troubleshoot traffic drops.

Monthly Volume is based on data specific to that month. Traffic that is based on Monthly Volume tends to be more volatile but helps you monitor seasonal search trends and popular topics.

If you would like to take into account search volumes in the past 12 months, pick the Average Volume option.

If you would like to take into account monthly/seasonal volume fluctuations, pick the Monthly Volume option.