Adobe "All Visits" Segment Data Discrepancy
Why would the Adobe Default "All Visits" segment result in a different count of "Unique Visitors" and "Visits" vs using "Page Dimension" and "Unique Visitors" and "Visits"?

Why would the Adobe Default "All Visits" segment result in a different count of "Unique Visitors" and "Visits" vs using "Page Dimension" and "Unique Visitors" and "Visits"?

You are correct about the column totals de-duplicating UVs and Vs... if that wasn't the case, the value of those columns would be higher, not lower.
But the key is looking at the "grand total" in your screenshot for table 1.
154,973 out of 155,223
and
185,648 out of 186,861
Your breakdown is by your "Page" dimension, this would mean that you have actions that are being tracked where those users aren't actually hitting pages. Do you maybe have embedded tracking on a Newsletter that is triggering as an "action" but not a "page view". Or maybe there are scenarios in your site where an action is triggered before the page view - and users trigger that, then bounce before the actual page view has occurred.
UVs and Vs will count against any hit, not just pages.... but when you are looking at Page data, the non-page data won't be included in the total; hence why the values are smaller than the Month or "All Visits" data.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.