Agreed, it's not a huge difference. There may be enough of a difference that the FSI may think that the slightly more similar vocabulary more or less makes up for French's slightly more complicated grammar such that it ranks French and Spanish as approximately equal in difficulty for English speakers.

Both languages come from Latin, but Spanish comes from an older version of Latin and so some innovations that occurred later are not found in Spanish. As an example: "to arrive", "arriver" (French), "arrivare" (Italian), but "llegar" (Spanish).