They have a set pool number, which you get at start.....some will decrease over time thru non-use, and others will increase thru use....but never more than the total amount of points you had at start.
I have had strength and spirit decay one point. I have had strength regain the one point lost in it.
You might decrease stats by a hundredth or a tenth, but only the whole number is displayed (rounding down in all cases). You can have 70 str and lose 0.1 or 0.01 and then it will resemble 69.9 or 69.99, but you'll see 69 only. Then if you had a stat at 70.1 you would think it is just 70.
From what I understand, orious13 has the right idea. It seems like every point you lose is gained somewhere else, so this theory that stats change in smaller increments seems to explain what is going on quite well.