Well, you eat different foods from day to day. And those different foods have different ratios of cal/fat/fiber. Which will result in higher or lower levels of cals for the same number of points. Right? So that should make sense. Also remember WW counts veg as 0 points, but that's not because veggies have no calories, just so few that they don't want to discourage us from eating them when they'd be a pain to keep track of. Or something like that. Same with some other "0-point" foods -- they actually do have cals so you'll see an impact on fitday but not with your points.
As for your other question, remember that 18 is your BASE TARGET. You should be going over this into your FPs on a regular basis, right? So if you're adding 5pts/day (if you spread them evenly), that's approximately an extra 250 cals (because I think I read someplace that 1pt is approximately 50 cals... correct me if I'm wrong!).
So right now I have 27 pts, which by the 50 cal estimate would put me at about 1300/day at my base target level. I just stuck my day into FitDay and yes, that's what comes out. Now considering I weigh over 200lbs that may seem low, but I eat the majority of my Flex Points throughout the week and therefore average closer to 1500/day.
I don't know if any of this helps... I'm kind of just making stuff up as I go and making guesses. I hope someone more informed than me comes along!