Confused by projected weight loss

Easiest just to ignore that.

It's meant, I suppose, to be a motivator but I think people's days are insufficiently uniform for it to have much meaning.

Your profile is not open for public viewing, so a general MFP user would not be able to give you an accurate response. Did you log exercise for the day so your net Calories are lower? Have you recorded a different weight in Check-in or Progress since the last time you completed your diary?
I would suggest that you open a Case with Customer Support on the MFP Help pages...
myfitnesspal.desk.com/customer/portal/emails/new

I've noticed that if you complete your diary on mobile it occasionally gives a lower number than on the webpage even when the content of calories consumed remains the same. I have no idea why though.

The "end of day" response that MFP gives is a very simple calculation. It takes the deficit or surplus of your current day's log (based on what your setup says your NEAT calorie requirements are), and projects that across the next five weeks. So if you had a deficit of 500 calories, it would estimate a 5 lb loss over that time. (500 * 7 * 5)/3500.

That's why it says "If every day was like today..."

It's not the most useful of calculations, since one day's activity is not what gives you results over time.