Because the scale is not a reliable way to measure body fat.
The water, solid mass, and glycogen in your system can cause the scale to fluctuate by several pounds within a day. It is entirely possible for you to have less body fat than you did 24 hours earlier, yet weigh more. And vice versa.
For example, here is an Excel chart showing my weight for the last several weeks, with a trendline. The trendline is a pretty good estimate of my actual body mass. As you can see, my scale reading is sometimes higher, sometimes lower. Throughout the time period on the chart, I was averaging the same calories in per week. If you compare March 25 with February 27, it would look like I'd lost 2 pounds, when in fact I'd gained 2.
Fat/muscle gain and loss are typically slow processes, and it's important not to read too much into the day-to-day changes on the scale.
Another less likely alternative is that you did not in fact have a 1000 calorie deficit. Estimates of calories burned, in particular, are very prone to error and individual variation, even if you're using a fairly reliable tool like a BodyBugg. If you maintain your weight for several weeks or a month, despite having what you think is a daily 1000 calorie deficit, then it's more likely you are overestimating calories burned and/or under-measuring calories taken in.
But the much more likely answer is: it's just water weight, not fat gain, and the scale will come back down within the next few days.
(and yes, I am deliberately adding weight right now)