I would guess that this perception is also based on the length of the text -- a period at the end of "Yes." is kind of weird for a text, versus a multi-sentence text ("Are you going? I'm heading there now.").
To test whether the period had become a social cue within the context of CMC, the researchers presented a small group (126 undergraduates — admittedly not representative of the entire global population, but at least fairly representative of the most prolific texters) with a series of exchanges framed as either text messages or handwritten notes.
...When that reply was followed by a period, subjects rated the response as less sincere than when no punctuation was used. The effect wasn't present in handwritten notes.
...In follow-up research that hasn't yet been published, they saw signs that exclamation points — once a rather uncouth punctuation mark — may make your messages seem more sincere than no punctuation at all.
"Texting is lacking many of the social cues used in actual face-to-face conversations. When speaking, people easily convey social and emotional information with eye gaze, facial expressions, tone of voice, pauses, and so on," Klin said in a statement. "People obviously can't use these mechanisms when they are texting. Thus, it makes sense that texters rely on what they have available to them — emoticons, deliberate misspellings that mimic speech sounds and, according to our data, punctuation."