Many of these rumors come from people being shown, or having described to them, work in progress (obviously). The people showing/telling them these things might be so enthusiastic about what they're working on, that it can seem to the person they're revealing these things to that these things are likely to be released as shown. One of Prosser's approaches is to convey that enthusiasm, but then (at least sometimes) toning it down by pointing out that these things may or may not look or work like he's been shown.
Many people have their hands on the elephant, and each describes what the overall beast must look like depending on what they're touching.