Finally got around to reading the 2009 smart brochure and found this on page 25 under standard equipment:
7. Multifunction display: ..."From a level of 1.3 gal the remaining fuel is indicated to an accuracy of 0.13 gal."
The first part of that sentence I interpret to mean the 1.3 gal reserve is included in the 8.7 total tank capacity, as opposed to being in addition to the 8.7 gal. YMMV.