If I had to guess, I think what knifegill was trying to say is that (for eg.) one cubic square inch of muscle weighs more than one cubic square of fat. Therefore, if you added muscle to your frame, but didn't lose the equivalent weight in fat, you'd show a gain. I think that's what he meant.
1. I hate that everyone uses digital scales, though I read that finding analog scales is getting tough. When I read fractions of pounds, my heart aches for the obsession this is creating.
2. 1.4 lbs is nothing. It's less than 1% of your total body weight. The fluctuation could be caused by a number of things from actual weight gain, menstrual cycle, an increase in sodium, etc.
3. While a lot of people don't like being a "slave" to the scale, the flip side of that is that if you weigh yourself often, you'll see how much your weight fluctuates even during the course of a day, and so not be totally freaked out by a pound and a half increase.
1200 calories is pretty low from some people's perspective, but I'm on board with it for losing - a lot of people disagree with that.
If you're not currently doing so, I'd encourage you to keep measuring spoons, measuring cups, and a food scale handy. If you're eyeballing portions, you may be consuming more than you think.
It isn't the IF that's making anyone gain weight. However, if you continue to gain weight on 1200 calories/day, I'd probably make an appointment with a doctor to see if you have metabolic issues.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine