That is a little weird about Fighting Def stacking with Expertise, if only because going Full Defense will give you a +4 Dodge bonus to your AC, whereas Expertise will give you (up to) +5, with another +2 on top of that. So, no attack at all, +4 AC. One or more attacks, +7. (The attacks you make won't have much of a chance of hitting, but then, they have more of a chance than no attack.)
Adding in Tumble and it gets weirder. With 5 Ranks you can get a Total Defense bonus of +6, but your Expertise is still +5 (assuming you have the BAB to back it up), and your Fighting Def bonus goes to +3, for a total of +8. With Two-Wpn Def, you add a Shield bonus of +2, so that makes it +8 for Total Def, and +10 for Expertise/Fighting Def.
I'm not sure I buy the logic of giving a character who attacks defensively more of an AC bonus than one who forfeits all attacks (including AoO's) in order to be completely defensive. For it to work out better, the Expertise character needs to be either a Fig3 or a Rog4, both fairly low level characters.
I would think it would make more sense for either Combat Expertise to not stack with Fighting Defensively, or for it to stack with Total Defense. Neither offends me. But having Fighting Defensively with Expertise be more of a bonus than Total Defense (with Expertise) seems odd, since the character would be getting less of a bonus when he's trying to be more defensive.