I don't work in the industry, but I did a bit of research on this recently. As I understand it, we're currently able to get several hundred Gb/sq. in. (bits, not bytes) with the perpendicular recording that we've been using since about 2005. Back in 2009, experts were predicting that perpendicular recording as it operates now would probably not be able to exceed 1 Tb/sq. in because of the interference problems at that density. Some of the major drive makers are pursuing alternatives and/or extensions to current recording technology. A couple of the major ones are Heat Assisted Magnetic Recording (HAMR, aka TAR), in which the physical/magnetic properties of the recording surface are altered with heat from a laser or other heat source, and Bit Patterned Media (BPM), in which bit boundaries are actually delineated using non-magnetic material (as opposed to the uniform recording surfaces of today). There are others like Microwave Assisted Recording and Domain-Wall Assisted Recording, but it sounds like these are less likely to be used, at least at first. Recently, there was also a story on Ars Technica describing an effort to combine HAMR and BPM in order to overcome some of the shortcomings of each. Common predictions seem to be that these new technologies would move the density limit up from about 1 Tb/sq. in. to something more like 10 Tb./sq. in. The growth rate has been in decline during this decade, despite the adoption of perpendicular recording, but the hope seems to be that it might rise back up to how it was in the 90s with the next advancement.

Playing psychologist might be fun, and you're probably right that people who lie in polls will do it in the most absurd way possible, but I don't think it's fair to conclude that this tendency accounts for the numbers we see here.

The "over 100" category in this poll is actually a very broad group. It includes not only those people who sent slightly over 100 letters, but also all others above that. Consider the people are in jobs where they send out hundreds, thousands, or perhaps even tens of thousands of letters per year. First, this group is less affected by the decline in personal letter writing, and second, every single one of these people is pigeon-holed into the same category of "over 100". It's no wonder this group is slightly larger.

GPG is a name chosen to describe the free version.
This sentence is neither informative nor funny.

No, GnuPG is not the same as PGP. GnuPG was in fact developed to replace PGP, both because PGP is covered by a non-commercial use only license, and (probably) because it by default incorporates the patented IDEA algorithm. Yes, PGP Freeware and GPG are both free and interoperable, but they are not the same thing.