What causes this when turning?

[outnumbered]

What causes the little defects in my final finish. Most of the plug is smooth and then I almost always have this little section of non smooth woodd. I have been turning with pine till I get better. I have even done some in mahogany and maple that also have this problem. Like I said most of it is smooth but I seem to have this small section that is rough by the growth rings.

Thanks,

Kevin