The list is no surprise. Their top returns can be classified into 3 categories:
1) Tablet cases/covers. Oftentimes they explicitly claim to fit the iPads, and also other 10.1/7" tablets, but end up too loose and the tablet slips out, and of course the straps aren't adjustable. Few people bring their tablet into the store to check, and it's likely a present and still in a box.
2) Devices which utilize radio waves. Interference by walls/furniture, and other devices, cause reception to vary widely. The overloaded 2.4GHz spectrum is making this gradually worse. For wireless audio, people have little tolerance for the signal cutting out. Remember 'antennagate'? A poor wifi antenna can make a tablet (or unlocked phone) hard to use.
3) Sticks of RAM. I was kinda surprised by this, although thinking back to how many unused sticks of RAM I own that my mobos just won't work with for various reasons, it shouldn't be too surprising. Some people likely get SODIMMs instead of DIMMs and vice versa, or the wrong speed, or the wrong DDR tech.
In brick and mortar, top electronics returns are phone chargers with the wrong plug (Lightning instead of micro-usb or vice versa), and $5 headphones whose wires snap after bending them twice. Tablets are next, followed by Wifi speakers. God, the tablets; the cheap ones are cheap enough to be unusable, but are expensive enough to warrant returning, so the return rate is ~75% on some of them. Printers were very frequently returned because the manufacturer tried to save 50cents by not including a USB-B cable; customers would complain it had no cable, and for some reason they don't have a dozen laying around their house like I do. Only including a black ink cartridge and no color (or vice versa) was another frequently given reason. If people weren't able to rip the packaging open and try it on, I imagine many smartphone cases would be returned; apparently noone knows what phone they have, and have to try to put the case on in order to figure out if it'll fit. At best, they know they have an iPhone, or 'a Samsung', but most often, it's e.g. 'a Verizon'. Most amusing return award: an HDMI cable returned for 'not working with a 3d signal' despite the packaging explicitly saying it did. Surprisingly, (small) TVs were almost never returned, I guess they really do encourage passivity.