Can you conduct the flight under para 15? If you can then great. If you cannot, then you need 3 crew for 13 hours, or 4 crew for 18.
Yes I agree with you, it would make more sense. But you have to keep in mind that 3 crew ULR ops doesn't use Table A or B. You can sign on any time of day and have a max FDP of 13 hours, where as a 3 crew Normal Ops has FDP restrictions based on whether you are acclimatized or unaclimatised. This goes back to my argument where these two tables, inlcuding para 15.4, are exclusive to Normal Ops flight (section 15). They were not meant to be used in conjunction with section 16. Even the old AoA AFTL flow chart was drawn in this manner.