Then the phenomenon could be stem from punctuation habits, as @bfinn says. Did you notice that my original comment doesn’t contain a sentence, by your standards?
What is a sentence anyway… is there something special about a period, as opposed to other punctuation marks? Many are available: the colon is a possibility; also its half-brother; and the comma,of course...also the ellipsis—even the mighty m-dash!
Question marks and exclamation points are dots with an extra bit. Ellipses may be multiple dots, but also indicate an uncertain end to the sentence. (Formal usage distinguishes ”...” for ellipses in arbitrary position and ”....” for ellipses coming after a full stop, but the latter is rarely seen in any but academic writing, and I would guess even many academics don’t notice the difference these days.)
Then the phenomenon could be stem from punctuation habits, as @bfinn says. Did you notice that my original comment doesn’t contain a sentence, by your standards?
Question marks and exclamation points are dots with an extra bit. Ellipses may be multiple dots, but also indicate an uncertain end to the sentence. (Formal usage distinguishes ”...” for ellipses in arbitrary position and ”....” for ellipses coming after a full stop, but the latter is rarely seen in any but academic writing, and I would guess even many academics don’t notice the difference these days.)