When you open a report for an owned Facebook profile, you'll see three different figures for reach: Page Reach, Post Reach, and Page Post Reach. These are three distinct metrics provided in Facebooks' Graph API.
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This figure is derived from daily page level reach. 

Daily page level reach refers to the page's reach on each day during the selected time period. It includes reach from posts, check-ins, ads, social information from people who interact with your Page and more. The total page reach figure is the sum of each day's reach. 

For example, if you selected a period of Jan 1 - Jan 3, total page reach would be calculated by adding up the daily reach on Jan 1, Jan 2, and Jan 3.

Pro Tip: Page reach can include reach from older posts. If someone logged into Facebook on January 1st and viewed a piece of content that was posted in December, that reach would be included in the page reach for Jan 1.
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This figure is derived from lifetime post level reach. 

Lifetime post level reach refers to the reach of a single post. The total post reach is the sum of the lifetime reach values for all content posted during the selected time period. 

For example, if you selected a period of Jan 1 - Jan 3 and during that time you posted 2 pieces of content, Post A and Post B, total post reach would be calculated by adding up the reach of Post A and Post B. 

Pro Tip: Post reach can include reach that occurred after the selected time period. If someone logged into Facebook on January 1st and viewed a piece of content that was posted in December, that reach would be included in the post reach for December. Therefore, it is normal for post reach to continue rising over time. 

This figure is derived from daily page level reach. 

It is identical to Page Reach, except it only includes reach from posts. It does not include reach from check-ins, ads, social information from people who interact with your Page and more.