No, this is not an inability to read the data. "Every 8th cell" is meant to mean that the cells on the page are numbered from first to last. They are then are stored internally and every 8th cell is XXXed out.
PDF is not a sequential document format... that is, for example, table cells can be added row-by-row or column-by-column or any other order (or no order whatsoever). So that is why sometimes "every 8th cell" does not look exactly as every 8th to human user - but if you count the number of cells and then count the number of XXXed out cells - you will see that the ratio is 1:8 for the page as a whole.
The Selection makes no difference. Cells are numbered based on their page position, not their position in a selected part of the document.
This article refers to Able2Extract and Able2Extract Professional.