It is apparently because, slog has a 16-235 (or something like that) range, whereas other picture styles have 0-255 range. The histogram shows range from 0 to 255 brightness values, but slog's highest value is just 235 - not perfectly white. Same with blacks.
That doesn't mean though, that your image is not overexposed. It actually is, but it is overexposed at 235 brightness value, not 255.
When first time shooting with a picture profile, that has a max value of 235, you have to be careful not to blow out highligts. Because if you don't know, that 235 is the max, you're looking at histogram and thinking - nice, the exposure is ok - but actually your highights are blown out at 235 and there's nothing you can do about it in post.