I couldn't come up with a better description of the phenomenon, sorry.

I would expect that when I see an airline publish as their base fare $X, that there is some fixed tax schedule (with perhaps graduated rates) applied to $X, plus PFC's depending on the departure, destination, and any connecting airports, plus the 9/11 security fee based on the number of takeoffs. However, I find that on WN, it doesn't quite work out that way. Instead, although the listed base fare is $X, I usually find that when I click through to the next page, the base fare is usually less than $X, and it appears that southwest.com is adjusting the base fare so that the total ends up being some specific pre-determined number.

E.g., SFO-LAS-SFO December 13 returning December 14. If I select an outbound flight with base fare $59 and a return flight with base fare $99, and click through to pricing, what I really get is:

Outbound: $54.88 (base) + $7.62 (US tax) + $4.50 (PFC's) + $2.50 (9/11) = $69.50
Return: $92.09 (base) + $10.41 (US tax) + $4.00 (PFC's) + $2.50 (9/11) = $109.00

It would appear that the base fare pricing is being adjusted so that the all-inclusive fare in each direction differs by the same $40 as the original published fare. (Although in this example, it doesn't work out precisely that way since there are different PFC's charged for SFO-LAS vs. LAS-SFO, so the price differential ends up being $39.50 instead of the original $40.00.)

Is that indeed what Southwest is trying to achieve?

This is non-intuitive to me, because it means for both the lower published fare segment and the higher published fare segment, the all-inclusive price is approx. $10.00 more than the published fare, whereas I would have expected to pay more taxes on the higher fare and therefore would expect the all-inclusive price for the higher published fare segment should be more than $10.00 than the all-inclusive price for the lower published fare segment.