There are a couple of reasons why the data may not match. 

Campaigns and ad groups are paused

The tool only includes data from active campaigns and ad groups. Even if an ad group was active during the selected date range, but is now paused, the optimization will not include data from it. This is because those entities are not currently actionable. 

Biddable Products and All Products

The default setting for the optimization is set to Biddable Products. This means only data from products groups for which bids can be changed is aggregated. In the All Products setting, only data from active campaigns and ad groups is included and data from Everything Else is not included. 

Impressions > 1

In the All Products setting, only data from product groups that have more than impression is aggregated.

Product groups or ad groups for each item id not available
The tool requires your campaign to be split at the item id level. This means that each item in the shopping or merchant feed has its own product group or ad group. This is the smallest entity that the feed can be broken down to, which makes it possible to aggregate data using any attributes. If the campaign doesn't have this structure, it is still possible to switch to the 'All Products' setting and use it as an analysis tool.

Attributes not available in the feed
The tool shows all the different attributes that by which it is possible to aggregate data, however, all these attributes may not be part of your product feed. You'll only see product groups broken up by the attributes that have values in the feed. 

For example, if you're selling TVs and you try to split the feed by gender you may not see results. Similarly, if you haven't defined custom attributes in the feed, you can't use them to split data. The screenshot below shows the different attributes that are available.