I think that is how it works. The manual setting is only a hack which helps to find the right combination of the base infill percentage (as set by the slider, like 100%,50%, 33%,25%, etc.) and the (new) infill extrusion width parameter so as to finally get the manually set infill percentage. So a setting of 90% is realized just by using the 50%-slicing with a very thick infill extrusion. In other words, the slicing is based on the base percentage value and the standard extrusion width, and the difference to the manually set percentage value is done by adjusting the infill extrusion width. You can see that when entering one of the possible base percentages manually, like 50%; then the infill extrusion width is always set to the standard extrusion width as specified on the left side (under layer thickness). When setting it manually to 49% instead, the base percentage of 33.3% is used with the infill extrusion width increased by a factor of 49/33.3. Apparently, the algorithm always picks the next lower base percentage and thus the infill extrusion width is never set to anything smaller than the standard extrusion width. Of course, when setting the infill extrusion width manually, values smaller than the standard extrusion width can be used.