While that can be a common procedure, it is only meaningful when a table is designed in way that supports the assumption that cross-summing has any purpose. In this case, that is not true.
The Tax Summary comprises rows summarizing transactions for unrelated tax codes. The purpose of each row is to yield the tax liability for each tax code. In the case of your Import custom tax code, the liability is correctly shown as zero. And since every transaction contributing to the liability was a purchase, everything shows up under the purchases columns except the reverse charged tax. Because it is opposite in sign, it shows up as a tax on sales.
The column sums are essentially meaningless on this report for your situation. In some circumstances (no reverse charged taxes), you could cross-sum, but that would only be a happy coincidence. They aren’t designed to cross-sum. So look at them as being there as a courtesy for other users in simpler situations.
The real issue here is terminology for column headings. Those have actually been changed over the years as the developer has searched unsuccessfully for headings that make sense under all circumstances. Various users have made competing suggestions. The only universally accurate headings would be some version of debits and credits, and those would be incomprehensible to almost everyone. So he settled on these.
I guess the ultimate message here is that the numbers on the report are accurate. Nothing is “fictitious.” Some of the figures just don’t relate to one another in the way you apparently expected them to. This is a report, not a spreadsheet.