The rhythm strip that is irregular but you see sinus beats

This could either be sinus arrhythmia or NSR with ectopics.
Some of these ectopics will be so early that they will be blocked - this creates the longer intervals seen on the ECG.
The surprising thing about atrial ectopics is that they can occur even before the qrst is finished! Thus, you can see them in the ST segment and superimposed on the T wave!