The Search Volume refresh occurs twice a year, usually in Jan/Feb and again in Aug/Sep. Since the refresh happens in the middle of the month, and keywords in the Research Grid are rank checked throughout the month, there are many keywords in which the rank check occurs prior to the Search Volume refresh. Research Grid keywords in Content Gaps will display the Search Volume total that was available at the time of the rank check. Topic Explorer's keywords, however, have Research Grid Search Volume updated regardless of the rank check status. 

 

Therefore after the refresh occurs, there will be a period of time when these keywords show different Search Volume numbers between Topic Explorer and the other Research Grid features. The following month, when the keyword is rank checked again it will use the updated Search Volume from the refresh and all features will match. Topic Explorer Search Volume historic data will stay the same after a refresh. Research Grid historic Search Volume data does not.

Research Grid backfills Search Volume data when it refreshes.

More details on the Search Volume refresh can be found at What is Average Search Volume and how often is it refreshed?