I know exactly what you are thinking but allow me to elaborate on that.
Zoro is the 1st person, Brook is the 8th person and so on. They are the Xth person to join but that isn't the number that Oda has assigned them. In an SBS and some other interview Oda has gone on record saying that Luffy is represented by the number 1, Zoro 2, Nami 3, Usopp 4, Sanji 5, Chopper 6, Robin 7, Franky 8 and then Brook 9. Many color spreads depict the Straw Hats with these numbers.
There are a ton of these but this is probably the best and easiest example. Some color spreads only show the numbers on some crew mates and not others. It would take me forever to find the SBS.
Because of where Jinbe came into the story, even if someone were to join before him he would still be the 9th Person for sure and 10th over all. As apposed to the 10th Person and 11th over all. My reasoning comes from Nami. Luffy had whole heartedly accepted her into the crew right after Zoro. Though she left and was later recruited officially after Usopp and Sanji, she still has her 2nd Person chapter during Arlong Park. Jinbe has already had 3 big Adventures with Luffy and was asked to join, when ever that happens, even if it's after some one else he'll be the 9th Person/10th member over all.
Edit: Actually I don't think I know what I'm talking about because I can't find Nami's 2nd person chapter anywhere.
Edit: Wait I do know what I'm talking about! Chapter 94 is The 2nd Person. VIZ interpreted it as something else and used some creative freedom and titled it The Other Villain. Who's the other villain? Nezumi?