This morning I asked question, and forgot a bit about it. However, a couple of hours later, I found a possible answer to it, and answered my own question. To my surprise, I saw that I had two upvotes on the question, but I never got any notification of that. I mean, there was no green "+5" or "+10" in the upper panel, where it should be.
Looking at my profile, those reputation points were awarded on my profile, and iirc, my total reputation was also increased. However, even when I opened the achievements tab, there was no notion of the increased rep. I could have made a screenshot of the awarded rep on my profile page, and missing notification in the achievements tab, but I didn't. However, I am sure there was delay (as in minutes to hours, not just mere seconds).
Now, just before I wrote this question, the green "+10" arrived in my achievements tab (hence I wrote this question). I never ever noticed this delay before. Was there something happening in the achievement system or something?