Yeah, it's just you. You should play more and get more rainbow 300s since they give better scores.
Since you said you were playing with DT or NC, try playing with no mods and slow down. The issue might be you're hitting way too early or too fast to get started but that would end in a bad result since you there's a chance you would misread the patterns and end up missing.
This can be checked in your hit error. Check your replays and see if your hit error goes to the left too much (that means you hit early) or goes to the right too much (late).