Are you using Karmic Dice (it's a game setting that defaults to on)? If so...yes. Karmic Dice is known to even out the odds of rolls such that the target DC (or AC) matters less. Enemies (and players) roll higher values against higher DCs. It also maybe does something to prevent you from succeeding or failing too many times in a row, adjusting the odds based on previous rolls.
Turn Karmic Dice off if you want "pure" rng.
If you've already turned Karmic Dice off, then how many rolls are you basing this on? If less than a few hundred, then it very well might just be rng combined with the human tendency to notice & remember bad things over good things. You need ~500 or more rolls to be able to declare anything about the rng on something with 20 different outcomes.
